I adapted this from a recipe I found on Food and WIne. It's so delicious on a warm summer night. I recommend using good quality Brioche buns for the burgers - it really does make all the difference. —ModernSwap
Wipe mushrooms clean with a damp paper towel. Remove stems and scoop gills out. Set the two prettiest mushroom caps aside. Take the 3rd (ugly) mushroom cap, slice it into large pieces and chop finely in a food processor. Place chopped mushrooms aside in a bowl.
Cut the bell pepper and shallot into large pieces, and place along with one garlic clove into the food processor and finely chop.
Heat grapeseed oil in a saute pan and saute bell pepper mixture until soft - about 5 minutes. Add chopped mushroom and continue to saute until most of the liquid is evaporated.
Place sauteed vegetables back in bowl and stir in the shredded smoked cheddar and one tbsp of the chopped dill. Season to taste with salt and cayenne pepper.
Heat a grill to medium heat (about 400) and grill the whole mushroom caps stem side down. This is very important – if you start with stem side up, the cap will collect juices and you’ll have to drain it before you can continue. Grill for 7 minutes, then transfer them to a plate and fill with sauteed cheese mixture. Place back on grill, filling side up, for another 5 minutes.
While the mushrooms are grilling, toast your brioche buns under the broiler for a minute or two until they are golden brown. Also its now time to make the yummy sauce that goes on the burgers - mix the mayo, remaining chopped dill and the last garlic clove (pressed or minced) in a small bowl. Season with pepper to taste and refridgerate until you are ready to serve.
When the mushrooms are done grilling, spread the toasted brioche buns with the garlic dill sauce, place the stuffed mushrooms on the buns, and top with fresh tomatoes and spinach.