NOTE: I used ground pork instead of ground beef, and the combination of gouda and cheddar cheese in this particular instance. But this slider is infinitely adaptable, switching grouda and cheddar with gruyere, monterey jack or even blue cheese.
To make the chili mayo: Toast the green chilis/jalapeno in a cast-iron/stainless steel pan over medium high heat until the skin is blistered and slightly charred. Remove the stems then add the chilis to a blender with garlic, chives, mayo, lime juice, salt, sugar and ground black pepper. Blend until smoothly pureed (you might have to stop the machine several times to press down the ingredients). Set aside.
To make the patties: Use the large holes on the grater, grate 1/2 of the cheese into large shreds. Then cut the other 1/2 of the cheese into small cubes. Mix ground pork or beef with worcestershire sauce, black pepper, salt and tabaso sauce evenly together. Using a round cutter, with a diameter slighter LARGER than the buns you are using (because patties shrink during cooking), measure and shape out 6 ~ 7 patties. Add about 1? thick of meat into the cutter and press down lightly to form flat edges, then release the patty from the cutter. Set aside.
To make the sliders: Heat up a non-stick pan over medium-high heat and add the unsalted butter and olive oil. Once the butter starts to foam, add the smashed garlic and thyme and give it a little swirl (thyme will start to pop). Brush the garlic and thyme to the side to make space for 2 patties (max 3 if using a larger pan)(do not crowd the pan with more than 3). Crack more freshly ground black pepper on the patties and carefully lower it onto the pan without breaking. Then DO NOT move it. Use a large spoon to base the top of the patties with the garlic/thyme oil, then keep the pan partially covered with a lid and cook the patties until you see the bottom crust starts to form and become nice and golden brown. Flip the patties and cook the other side the same way (at this point you can remove the garlic to prevent burning). You should have browned cheese-crust on both sides of the patties. If you want to finish cooking the rest of the patties, add fresh garlic and thyme to the pan (and more oil if needed) and repeat.
Cut the sweet potato and browned butter rolls open (recipe follows). Apply with generous amount of whole grain mustard on the bottom, then top it with one patty and about 1 tbsp of the green chili mayo for each slider. Top it with paper-thinly sliced shallots and the top bun. Serve immediately.
If you are anticipating re-heating of the sliders, only add the green chili mayo at the last min before serving otherwise mayo would break if microwaved.
SWEET POTATO AND BROWNED BUTTER ROLLS:
Melt the unsalted butter in a sauce pan and keep heating/swirling over medium heat. The butter will foam and sizzle for a few min, then the sizzling will gradually reside and the browned milk solids will start to form. Turn off the heat completely. Add the browned butter to the cold milk and stir slightly to bring the temperature down a little. The mixture should be warm to the touch but NOT HOT (when in doubt, measure with a thermometer for 110ºF/43ºC). Add the instant dry yeast and let it dissolve for 10 min.
In a stand-mixer with a dough-hook attachment (or by hand), mix the mashed sweet potato, bread flour, sugar and salt. Then add the browned butter/milk/yeast mixture and knead on medium-high speed until the dough is smooth and elastic, approx 6 ~ 7 min. The sweet potato should be completely broken down into the dough. If not, break it with your finger tips until there’s no visible pieces. The dough should be soft but not overly sticky. If it seems dry and tough, add 1 tsp of milk at a time to adjust. Knead for another 2 ~ 3 min after adjustment.
Cover the bowl with plastic wrap and a kitchen towel. Leave at a warm place in the kitchen for 1:30 ~ 2 hours to proof until doubled in size.
Scrape the dough out of the bowl and punch the air out. Fold the dough over itself a few time, then divide it into 12 equal portions. Keep tucking the sides of the dough under itself until you have a smooth-surfaced ball. Set them over a baking sheet lined with parchment paper, then cover with plastic wrap and a kitchen towel. Let them proof under room-tempt again until poofy and expand approx 80% more (not fully doubled).
Preheat the oven on 400ºF/200ºC.
Brush each rolls with egg wash and sprinkle with a few flakes of sea salt. Bake in the oven until golden browned on all sides, approx 20 min.