Heat a large skillet over medium heat. Add the olive oil to the skillet then add the onion, mushrooms, garlic, and a pinch of salt. Sauté until the onion turns translucent, about 10 minutes, stirring occasionally.
Meanwhile, make the flax eggs. In a small bowl, combine the flaxseed meal and water and let it sit until it “gels” up, about 5 minutes. While the flax egg is setting up, add the drained and rinsed lentils and garbanzo beans to a large bowl. Smash them with a fork until no large pieces are left.
Add the oats, milk, ketchup, Italian seasoning, salt, and pepper to the bowl. Add the sauteed onion and mushrooms to the bowl and stir to combine well. Spray a loaf pan with cooking spray and then spoon the mixture into the pan. Bake the loaf in the oven for 30 minutes.
While the meatloaf is cooking, make the topping. Whisk together the ketchup, sugar, and apple cider vinegar in a bowl or measuring cup. When the loaf is finished baking, remove from the oven. Spread a layer of the topping sauce over the entire loaf and then bake for another 15 minutes. Remove the pan from the oven and let it cool so it sets, for about 10-15 minutes before serving.
Notes
Tip: Place the loaf pan on a baking sheet after adding the sauce on top in case it drizzles over the edges in the oven.