Preheat your oven to 400 degrees. This step is crucial as it ensures even cooking and helps the loaf develop a nice crust.
In a large skillet, add a 1/4 cup of water and bring it to a gentle simmer. Once the water is warm, toss in the diced onion and sauté until it becomes tender, about 5 minutes. If you find the water evaporating, feel free to add more. This process helps to draw out the flavors of the onion.
Add the carrots, celery, and mushrooms to the skillet. Continue sautéing until the mushrooms have released their liquid and become soft. This will take about 8 to 10 minutes. Drain the mixture well and set it aside; we want to keep our loaf from becoming soggy.
In a food processor, combine the lentils, tomato paste, liquid aminos, balsamic vinegar, oats, almond meal, and dried oregano. Pulse the mixture until the ingredients are well combined but still have some texture. This mixture forms the heart of the loaf.
Now, add the sautéed veggie mix to the food processor. Pulse again until everything is just combined and chunky. You want to see bits of vegetables without them being fully pureed. Transfer this mixture to a large mixing bowl.
Lightly spray a 5 x 7-inch loaf pan with non-stick cooking spray to prevent sticking. Pour the mixture into the pan, then use your hands to form it into a loaf shape. Make sure it's compact for better slicing.
Bake the loaf in the preheated oven for 35 minutes. This step helps meld all the flavors together and gives the loaf a delicious crust.
After 35 minutes, carefully remove the loaf from the oven. Spread the ketchup on top and return it to the oven for an additional 15 minutes. This creates a sweet glaze that enhances the flavor.
Once baked, remove the loaf from the oven and let it sit for about 10 minutes before slicing. This resting time allows it to set, giving you clean slices every time.