Mix the mascarpone, dill, 1 teaspoon lemon zest, and 2 teaspoon lemon juice in a medium bowl. Season with salt, pepper, and more lemon juice or zest to taste. Using a sharp vegetable peeler, peel the fennel into long, thin strips by pressing firmly against it; season the strips with salt.

To assemble, spread the toasts with some of the mascarpone and then cut each toast into four even squares. Top each square with a couple of pieces of the fennel, a curl of the salmon, a dill frond, and a few grinds of black pepper.

Make Ahead Tips: You can make the mascarpone spread and prep the fennel several hours in advance. Keep both refrigerated; bring to room temperature before assembling.

Adjust an oven rack to 6 inches from the broiler and turn the broiler on to high. Set the bread on a baking sheet, brush one side with the melted butter, and season with salt and pepper. Toast the bread until it's golden brown and crisp on top, about 1 to 2 minutes. Flip and cook the other side until golden, about 1 minute. While the bread is still hot, slice off the edges. Let cool slightly. Spread with your choice of toppings before cutting into squares or triangles.

Make Ahead: Toasts can be made up to a day ahead; store them in an airtight container.