For the crostini: Preheat the oven to 375 degrees F. Arrange the baguette slices in a single layer on a large baking sheet. Mix the oil, garlic, oregano and fennel seeds in a small bowl. Brush the oil mixture over the top of the baguette slices. Sprinkle lightly with pepper. Bake the baguette slices until lightly toasted but still slightly soft, 12 to 15 minutes.
For the fondue: Mix together the beer and mustard in a small bowl. Heat the oil over medium-high heat in a large nonstick skillet. Add the sausage, onions, fennel seeds, salt and pepper. Break the sausage into smaller pieces with a wooden spoon and cook until browned and cooked through, about 10 minutes. Stir in the flour. Add the beer mixture and bring to a boil. Reduce the heat to medium-low. Gradually add the Cheddar, fontina and Parmesan, stirring constantly, until the cheese is melted and smooth. Keep warm over low heat. If the fondue becomes too thick, gradually stir in extra beer, 1 tablespoon at a time.
Serve the fondue with the crostini.