Preheat oven to 400 degrees F. Place the garlic in a piece of foil on a baking sheet. Drizzle with a little olive oil and sprinkle with salt and pepper. Seal the foil. Place the fennel quarters on the same baking sheet. Drizzle them with olive oil, and sprinkle with salt and pepper. Bake for 40-45 minutes, turning the fennel once or twice, until well caramelized.
Squeeze the garlic cloves out of their skin and into a food processor. Add the fennel, beans, rosemary, lemon juice, 1 1/2 teaspoons salt, and pepper to taste. Process until well combined, but not a completely smooth puree.
Lower the oven to 350 degrees F. To prepare the crostini, cut a baguette into 1/4 to 1/2 inch thick slices and place on a baking sheet. Brush with olive oil and sprinkle with salt and pepper. Bake for about 15 minutes, until the bread is a light golden brown.
Right before serving, put about a tablespoon of the fennel mixture on each crostini. Garnish with a fennel frond.