Preheat your oven to 375°F and prepare a baking dish by lining it with foil or lightly greasing it with oil.
In a mixing bowl, combine the Greek yogurt, whole grain mustard, lemon juice, onion powder, thyme, sea salt, and black pepper. Stir until smooth and well blended.
Arrange the salmon fillets in the prepared baking dish, skin side down. Drizzle avocado oil over the fillets and coat each with the yogurt mixture.
Bake in the oven for 15-20 minutes, or until the salmon is cooked through and flakes easily with a fork, reaching an internal temperature of 145°F.
Allow the salmon to rest for a few minutes before serving and garnish with fresh lemon slices.