Heat olive oil in a 10-inch cast-iron skillet over medium-high heat. Add salmon to skillet; cook for 7 to 8 minutes or until fish flakes easily with a fork (145°), turning halfway through. Remove salmon from skillet; set aside.

Melt butter in the skillet over medium heat. Add onion and cook until softened. Whisk in flour, dried thyme, and garlic; add tomatoes. Cook until fragrant. Remove skillet from heat; stir in wine and Dijon mustard. Return pan to low heat. Cook 5 minutes or until mixture starts to thicken, stirring occasionally.

Slowly stir in heavy cream, lemon zest, salt, and black pepper. Cook and stir over low heat until mixture begins to thicken. Remove from heat; add spinach and stir until wilted. Return salmon fillets to pan and cook over low heat 2 minutes or until salmon is heated through. Garnish with fresh thyme, if desired.