Melt butter with 2 Tbsp. olive oil in a large skillet over medium heat. Add fennel and onion and cook, stirring occasionally, until mixture begins to brown, 20 to 25 minutes. Pour in wine, reduce heat to medium-low, cover and cook, stirring occasionally, until golden brown and soft, 8 to 10 minutes longer. Season with salt and pepper.
Preheat oven to 350°F. Brush both sides of baguette slices with remaining 3 Tbsp. olive oil and place on a baking sheet. Bake until edges are golden, 6 to 8 minutes per side.
Spoon 1 generous Tbsp. of topping onto each toasted baguette slice. Garnish with reserved fennel fronds, if desired.