Combine the tomatoes, almonds, basil, lemon juice, garlic, and nutritional yeast in your blender or food processor. Puree until it's as smooth as you can get it. If things aren't blending up nicely, add water 1 tablespoon at a time until you get a nice, smooth puree. I used a very old blender to test this recipe, and it took about 2 tablespoons of water. If you have a high-speed blender, you probably wouldn't need any.
Stir in the salt and pepper, to taste.
Arrange the mushrooms on a baking sheet greased with olive oil or one lined with a silicone baking mat. Generously fill each mushroom cap with the tomato mixture.
Bake for 20 minutes. You can serve these cold or warm.