Written and photographed by Sabrina Russo
Add mushrooms to a bowl with oil, vinegar, salt and pepper. Marinate at room temperature for about 30 minutes.
Divide meat into 4 patties slightly wider than buns. Press a slight indent in the center to account for puffing during cooking.
Grill mushrooms and onions over medium-high heat for 3-5 minutes per side. Slice mushrooms.
Split buns. Place on grill cut-side down. Toast 1-2 minutes on grill over medium high heat.
Season burgers. Cook about 2-2 ½ minutes per side for medium-rare, or 1-3 minutes more for medium to well-done.
Do first, if making! Emulsify egg yolks with oil. Add garlic, lemon, vinegar and tarragon. Use store bought mayo for a quick version.