Bring a small pot with lentils and 2 cups water to a boil. Reduce to a simmer and cook until very tender and breaking down, 11-13 minutes. Transfer to a wire-mesh strainer, rinse, and drain well, pressing down on lentils to extract as much liquid as possible. Place drained lentils to a mixing bowl. While lentils cook, prepare ingredients.

Finely chop mushrooms. Halve and peel onion. Slice two rounds from onion and cut remaining into a fine mince. Finely chop one bun (reserve remaining for burgers). Remove one leaf from romaine and cut into thin strips. Hold remaining romaine heart at root end and chop coarsely. Core tomato and cut into 1/4" rounds. Cut rounds into half-moons. Combine mayonnaise and half the seasoning blend (reserve remaining for patties) in another mixing bowl.

Heat a large non-stick pan over medium-high heat. Add 2 tsp. olive oil, mushrooms, diced onions, and a pinch of salt and pepper to hot pan. Cook until vegetables are tender and brown, 4-6 minutes. Remove from burner and transfer to bowl with lentils. Wipe pan clean and reserve.

Add chopped bun, half the flour, and remaining seasoning blend to bowl with lentils and vegetables, and use your hands to mash together. Starting with half, add remaining flour as necessary to form patties that hold together. Form into two patties, 31/2-4" in diameter. Return pan used to cook vegetables to medium heat. Add 2 tsp. olive oil and patties to hot pan and cook until brown and crispy, 3-5 minutes per side. Remove from burner. Reserve pan; no need to wipe clean.

Return pan used to cook patties to medium heat. Add remaining buns, cut side down, and toast until browned and crispy, 1-2 minutes. In another mixing bowl, combine seasoned rice vinegar, 2 tsp. olive oil, and a pinch of salt and pepper. Toss with chopped romaine and half the tomatoes (reserve remaining for burger).

Divide sauce between bottom buns. Place bottom bun on plate and top with patty, onion round, remaining tomato, and romaine strips. Cover with top bun. Serve with salad on the side.