Sprinkle 1 teaspoon each salt and pepper on the salmon.
Heat the oil in a pan over medium high heat. Add the salmon, skin side down, cooking until the skin is crispy, about 5 minutes. Flip and cook on the other side until fish is cooked to desired doneness. Remove from pan, set aside.
Melt the butter in the pan, scraping the brown bits from the bottom.
Stir in the onions, cooking until starting to brown, about 10 minutes. Stir in the garlic, cooking for about 30 seconds.
Sprinkle with cornstarch, stir to combine.
Pour in the beer, scraping to deglaze the pan. Allow to simmer until reduced by about half.
Stir in the cream.
A handful at a time, stir in the cheese. Stir until completely melted before adding more cheese.
Stir in the garlic powder and remaining 1 teaspoon salt and pepper.
Stir in the spinach, cooking until wilted. Remove from heat, stir in the tomatoes and basil.
Return the salmon to the pan, serve warm.