Melt 1/2 cup butter in a large skillet over medium-high heat; cook and stir onion and 1/2 of the garlic, breaking onion into rings, until softened, 5 to 10 minutes. Add salmon to onion mixture and cook until salmon is about 1/4-cooked, about 2 minutes.
Pour wine and cream over salmon and cook, stirring lightly and flipping salmon halfway through, until liquid is reduced by 1/2 and salmon flakes easily with a fork, 5 to 10 minutes.
Melt 1 tablespoon butter in a skillet over medium heat; cook and stir remaining garlic until fragrant and golden, 1 to 3 minutes. Add spinach and cook until spinach is wilted, 3 to 4 minutes.
Place spinach on 2 serving plates, making a well in the center of each. Arrange each salmon steak in the center of the well and pour sauce over each. Garnish salmon with onion.
