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 3 minutes. Add carrots, celery, and garlic; cook and stir until tender, about 4 minutes.
Combine remaining 2 tablespoons olive oil, onion mixture, chickpeas, bread crumbs, almond milk, 2 tablespoons ketchup, flaxseed meal, soy sauce, salt, pepper, and sage in the bowl of a food processor. Pulse until just combined; do not overblend. 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.