cornstarch slurry1 tablespoon each of cornstarch and water, mixed into thin paste (optional)
Heat 2 tablespoons of butter in a large heavy bottomed skillet over medium high heat.
Season flesh side of salmon with salt and pepper and add to preheated skillet skin side down. Cook for 4-5 minutes per side until golden brown on each side and cooked to your liking. Transfer salmon to a paper towel lined plate while you make the sauce.
Add two tablespoons of butter to the same skillet you cooked the salmon in. Once butter is melted, add onion and saute until translucent, about 2-3 minutes. Add garlic and tomatoes and cook until garlic is fragrant, about another minute. Add broth to pan and stir to break up any bits stuck to the skillet. Add half and half and bring to a boil.
Reduce heat to medium and season sauce with salt, pepper, garlic powder, oregano, basil, thyme and cayenne pepper. Add balsamic vinegar and continue to cook, while stirring, until sauce begins to thicken, about 5 minutes. Add spinach and cook until spinach is wilted. If you prefer your sauce to be a bit thicker, you can add the corn starch slurry at this time. Add more salt and pepper to taste.
Transfer salmon back to skillet with sauce, skin side down and spoon sauce over salmon. Remove skillet from heat and serve salmon immediately.
You may use skinless salmon as well if you prefer. I find that leaving the skin adds more flavor to the dish. Not to mention, the crispy skin is delicious!