Lightly season salmon filets on each side with salt and pepper.
In a large skillet, melt 1 Tbsp butter over medium heat. Add salmon to the pan, skin-side-up and cook for about 4 minutes. Flip and cook for another 4 minutes skin-side-down. Remove salmon from pan and set aside on a plate.
Add remaining 1 Tbsp butter to the skillet. Add onion and cook for about 5 minutes. Add garlic and cook for another 2 minutes, until garlic is fragrant and onions are translucent.
Add grapefruit zest and juice to pan and bring to a simmer, scraping up the browned bit from the bottom of the pan with a spatula. Once simmering, slowly pour the cream into the pan. Stir and let sauce thicken a bit.
Add salmon back to pan to heat through, spooning sauce over the salmon.
Serve salmon with brown rice, quinoa, or pasta or simply with a vegetable side.