To make pasta sauce, in a large pan, over medium heat, sauté chopped onion and garlic with some oil. Sauté until soft.
Add spinach, crushed tomatoes, white miso paste, tomato paste, water, and oregano. Stir occasionally.
Simmer uncovered for 10-15 minutes, stirring occasionally.
Stir in cooked green lentils, salt and black pepper to taste. Let simmer for a few minutes longer.
Preheat oven to 230°C (446°F).
To make the white sauce, melt butter in a saucepan over low heat.
Stir in flour and whisk constantly. Cook for about 2 minutes, but don't brown.
Stir in oat milk gradually, constantly stirring. Cook over low heat until sauce begins to thicken.
Season with salt and black pepper to taste.
To assemble your lasagna, spread some pasta sauce on the bottom of casserole dish. Place 4 noodles on the top. Spread a layer of pasta sauce over the lasagna noodles and top with noodles. Repeat layering two more times to create three complete layers. Top with lasagna noodles.
To finish, spread the white sauce over the top layer of noodles.
Bake in the oven for 30-35 minutes, until golden brown.
Let it sit for a few minutes before cutting.