Heat olive oil in a large skillet over medium heat. Add shallots and garlic and cook for 3 minutes until they begin to soften. Add garlic and cook for 1 minute more. Add spinach, sun-dried tomatoes, red pepper flakes, 1/2 teaspoon salt and 1/4 teaspoon pepper and cook an additional 2 minutes. Remove from heat and let cool approximately 10 minutes. Add ricotta and stir to combine. Season with additional salt and pepper, to taste.
Using your hands, pack approximately 1/2 cup spinach mixture on top of each salmon fillet, forming mixture to the shape of the fillet. Place fillets on a rimmed baking sheet or glass baking dish and bake for 15 minutes, until salmon is cooked through.