Preheat the oven to 425°F. Line a baking sheet with parchment paper.

Heat the oil in a medium skillet over medium heat. Add the onion and sauté until translucent, about 5 minutes. Add the garlic and continue cooking until the onion is golden, 2–3 minutes more.

Add the mushrooms to the skillet and cover. Cook until wilted, about 5 minutes. Drain any liquid from the skillet, then transfer the mushroom-onion mixture to a food processor along with the cooked lentils. Pulse on and off until the mixture is finely chopped but not pureed.

Transfer the mixture to a large bowl. Add the cooked quinoa, marinara sauce, oats, smoked paprika, mesquite seasoning, garlic powder, and onion powder. Season with salt and pepper. Mix until evenly combined.

Lightly oil a round ½-cup measuring cup. Scoop a level portion of the mixture and invert onto the prepared baking sheet, tapping firmly to release. Flatten to about ½-inch thickness with the bottom of the cup. Repeat with remaining mixture.

Bake for 15 minutes, then carefully flip each burger and bake for another 10–15 minutes, until golden and firm on each side. Let cool for 5 minutes before serving.