Pat the salmon fillets dry with paper towels and season both sides with salt and black pepper.
Heat olive oil in a large skillet over medium-high heat.
Place salmon in the skillet skin-side up and sear for 4-5 minutes until golden brown.
Flip the fillets and cook for another 3-4 minutes until cooked to desired doneness; remove salmon from the skillet and set aside on a plate.
In the same skillet, melt the butter over medium heat.
Add the diced shallots and sauté for 2 minutes, then add the minced garlic and cook for 1 minute until fragrant.
Pour in the pureed roasted red peppers and heavy cream, stirring to combine.
Bring the sauce to a gentle simmer and whisk in the Parmesan cheese and thyme.
Reduce heat to medium-low and simmer for 3-5 minutes until the sauce thickens slightly.
Stir in the baby spinach until just wilted.
Add the lemon juice and return the salmon fillets to the skillet, spooning sauce over the fish to warm through.
Garnish with fresh basil and serve immediately.