Using paper towels, pat both sides of the salmon fillets dry; season with 1 teaspoon salt and 1/2 teaspoon pepper.
Melt butter in a large cast iron skillet over medium heat. Add salmon fillets to the skillet, skin side up, and cook until golden brown and a crust has formed, about 4-5 minutes. Using a fish turner, flip, and cook for an additional 4-5 minutes, or until desired doneness. Remove and keep warm.
Reduce heat to low; add garlic and shallots, and cook, stirring frequently, until fragrant, about 2 minutes.
Whisk in flour until lightly browned, about 1 minute.
Stir in wine, scraping any browned bits from the bottom of the skillet.
Stir in chicken stock and half and half until slightly thickened, about 2-3 minutes; season with salt and pepper, to taste.
Stir in spinach until wilted, about 1-2 minutes. Return salmon to the skillet; sprinkle with dill.
Serve immediately with lemon wedges.
Notes
*Chicken stock can be used for white wine as a non-alcoholic substitute.*Half and half is equal parts of whole milk and cream. For 1 cup half and half, you can substitute 3/4 cup whole milk + 1/4 cup heavy cream or 2/3 cup skim or low-fat milk + 1/3 cup heavy cream.