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.
Pour in wine and cook, stirring lightly until liquid is reduced by 1/2, 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.
Combine spinach mixture with other sauce and serve over the top of your favorite meat or vegetarian food.