Melt the butter in a large skillet with a tight-fitting lid over medium heat; add orange slices, basil and onion and sauté until fragrant, about 1 minute.
Add garlic and cook until onion is translucent, 5 to 7 minutes. Add broth to the skillet and whisk up all of the browned bits from the bottom of the pan.
Add fish to the skillet then cover and cook for 8 to 10 minutes. Stir in spinach; cook until wilted, 2 to 3 minutes. Serve fish topped with sauce!