Preheat the oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.
Place the baby spinach and halved cherry tomatoes on the baking sheet, drizzle with olive oil, and toss to coat. Spread them into an even layer to serve as a bed for the fish.
Pat the salmon fillets dry with paper towels to ensure proper searing. Place the fillets on top of the vegetables.
Season each fillet lightly with kosher salt and black pepper.
Divide the Boursin cheese into four equal portions and spread or crumble it evenly over the top of each salmon fillet.
Bake in the center of the oven for 12 to 15 minutes, or until the salmon reached an internal temperature of 145°F (63°C) and flakes easily with a fork.
Optional: Switch the oven to broil for the final 1 to 2 minutes to lightly brown the cheese topping, monitoring closely to avoid burning.
Remove from the oven and immediately top with lemon zest, lemon juice, and fresh parsley before serving.