Preheat oven to 375 degrees F (190 degrees C). Grease a 9-inch loaf pan with cooking spray.
Heat 1 tablespoon oil in a skillet over medium heat; stir in onion. Cook and stir until the onion has softened and turned translucent, about 4 minutes. Add mushrooms and garlic; cook and stir until tender, about 3 minutes.
Combine remaining 2 tablespoons olive oil, mushroom mixture, cooked lentils, bread crumbs, almond milk, 2 tablespoons ketchup, flaxseed meal, soy sauce, salt, pepper, and Italian seasoning in the bowl of a food processor. Pulse until just combined; do not overblend and work in batches if needed. Press mixture into the prepared loaf pan. Brush top with remaining 2 tablespoons ketchup.
Bake in the preheated oven until golden, about 45 minutes.