Preheat oven to 350°. Wipe mushroom caps with a damp cloth and remove the mushroom stems. Place caps on a baking tray and set aside.
Heat skillet on medium and add sausage. Cook crumbling meat with spatula until no longer pink. Remove sausage with a slotted spoon and set aside.
Add onions and poblano pepper to the skillet and cook until tender about 5 minutes. Add garlic to the skillet and cook for 2-3 minutes. Return sausage to the skillet and add the tomato sauce and cayenne pepper. Cook for another 3-4 minutes. Add spinach and basil leaves and cook until spinach wilts.
Remove skillet from heat and fill mushroom caps with the sausage mixture.
Bake for 20 minutes and serve.
adapted from Paleo Recipes by paleoholic
Makes a great substitute for pizza.