Pat salmon fillets dry and season both sides with salt and black pepper.
Heat 1 tbsp olive oil in a large skillet over medium-high heat.
Place salmon fillets skin-side down and cook for 4-5 minutes until crispy.
Flip salmon and cook for another 3-4 minutes until cooked through; remove from skillet and set aside.
In the same skillet, add butter and remaining olive oil; reduce heat to medium.
Add minced garlic and sauté until fragrant, about 1 minute.
Add fresh spinach and red pepper flakes (if using), tossing until spinach wilts, about 2-3 minutes.
Season spinach with salt and pepper to taste.
Serve salmon fillets atop the garlic spinach and garnish with lemon slices.