Heat 1 tbsp olive oil in a cast iron skillet or heavy bottomed pan over medium high heat. Salt and pepper all sides of salmon filets. Add to pan and cook 3 minutes. Do not move the fish. Carefully flip and cook another 3 minutes (the salmon will be cooked a little longer in the sauce so do not worry if it still looks rare). Remove from pan and set aside. Add asparagus and allow it to blister in the pan, only stirring occasionally. Add serrano pepper and cook another minute to char. Remove veggies from pan. Turn heat down to low. Add 1 tbsp olive oil and 1 tbsp butter to pan. Add shallots and saute 2 min, stirring constantly. Add garlic and cook 1 min. Add lemon slices and cherry tomatoes and cook 1 min, scraping any browned bits from bottom of the pan. Add remainder of butter and heavy cream and stir. Add spinach, herbs and lemon juice and stir together. Add 1 tsp salt and 4 turns of freshly ground pepper. Return asparagus to sauce. Place salmon filets in sauce. Cook 3-5 minutes or until salmon reaches preferred doneness.