Preheat oven. Pat salmon fillets dry with a paper towel, then place on a rimmed baking sheet. Season with Kosher salt and freshly ground black pepper. Set aside.
Heat 2 Tbsp olive oil in a skillet over medium heat. When hot, add shallot and cook until softened. Add chopped garlic and crushed red pepper flakes, then stir. Cook 1 minute more. Add baby spinach, chopped sun dried tomatoes, basil, lemon zest, and remaining Kosher salt. Stir well.
Transfer the spinach mixture to a mixing bowl, then allow to cool slightly. Add ricotta cheese, then mix very well.
Carefully divide the ricotta mixture between each piece of salmon, then pile on top, covering the entire fillet.
Transfer to the preheated oven and cook. Serve immediately with remaining basil, lemon wedges, and a pinch of flaky sea salt.