Stuffed Pasta Shells Recipe with Onions

2 tablespoons extra virgin olive oil 1 medium yellow onion, chopped 2 lb Italian sausage 16 jumbo shells 15 ounces whole milk ricotta 1 large egg 2/3 cup parmesan, finely shredded 1/3 cup fresh basil, finely chopped 2 cloves of garlic, minced 1 teaspoon Italian seasoning salt and black p