Party hors d'oeuvres can be a challenge, but here's a sure-fire crowd pleaser that doesn't require hours of work. Both the crab mixture and French bread croutes can be made up to 4 days in advance. Spread the crab on the toasts a few hours in advance of cooking, then pop the tray under the broiler when you are ready to serve.
Place the crabmeat, garlic, lemon zest, mayonnaise, and Gruyère in a bowl and stir to blend. Add the dill and cayenne pepper to taste. Cover with plastic wrap and refrigerate.
Position a rack in the middle of the oven and preheat to 325 degrees F.
Arrange the slices of bread in a single layer on a baking sheet; you may need to do this in batches.
Bake the bread until the toasts are firm and dry, about 15 minutes. Watch the toasts carefully so they don't overcook or burn. Remove the tray to a wire rack and let the toasts cool. The toasts can be stored in a tightly covered container for up to 4 days.
To cook, set a rack 5 or 6 inches from the broiler and preheat the broiler. Spread some of the dilled crab mixture onto each toast. If desired, lightly sprinkle the top of each toast with paprika. Arrange the toasts in a single layer on a baking sheet and cook until the crab mixture puffs and bubbles, 2 to 3 minutes. Serve immediately.