Add the butter to a very large skillet. Set on the stove over medium heat. When the butter has melted, add the onions. Season with a big pinch of salt. Cook, stirring occasionally and lowering the heat as needed, for about 15 minutes, or until they’re soft and lightly golden. (It’s okay if they stick to the bottom of the pan a little, but you don’t want the bits stuck to the bottom of the pan to burn—they’ll come in handy soon.)
While the onions are cooking, make the bread crumbs. Tear each English muffin into chunks, then add those to a food processor. Pulse until very fine.
When the onions are done, turn off the heat, then add the ¾ cup water to the pan. Use a spoon to scrape up the browned bits stuck to the bottom of the pan. Remove from the heat and let cool slightly.
Heat the oven to 350°F. Line a rimmed sheet pan with foil.
Add the bread crumbs to a large bowl, along with the eggs, 2 tablespoons ketchup, 1 tablespoon salt, and black pepper. Stir to combine. Add the ground beef and stir until mostly combined (you can use your hands, too!). Add the cooled onion-water mixture (it’s okay if it’s slightly warm still—you just don’t want it hot). Don’t overmix, which can make the meatloaf tough.
Dump the meatloaf mixture onto the lined sheet pan. Form into a loafish shape, roughly 9x5-inches. Add lots of ketchup and rub all over (you can use a brush but I just use my hands).
Bake for 50 minutes to 1 hour, rotating halfway through, until the center is firm to the touch and reads 150° to 155°F on a thermometer. (I like to start checking at 45 minutes just to be safe.) Let rest for about 15 minutes before slicing.
Emma is a writer and recipe developer at Food52. Before this, she worked a lot of odd jobs, all at the same time. Think: stir-frying noodles "on the fly," baking dozens of pastries at 3 a.m., reviewing restaurants, and writing articles about everything from how to use leftover mashed potatoes to the history of pies in North Carolina. Now she lives in Maplewood, New Jersey with her husband and their cat, Butter. Stay tuned every Tuesday for Emma's cooking column, Big Little Recipes, all about big flavor and little ingredient lists. And see what she's up to on Instagram and Twitter at @emmalaperruque.