Cook cauliflower in a pot of boiling water until just tender and then drain. Cook the fennel in salted boiling water until just tender, not mushy and drain.
Preheat oven to 375 F. While the vegetables cook, melt 1/4 cup butter in a heavy saucepan over medium low heat and whisk in flour. Continue to cook for about 2-3 min. Slowly add in the milk while whisking and bring to a boil. Turn the heat down and simmer the sauce, stirring occasionally for 8 min. Remove from heat and stir in cheese, salt and pepper. Pour over the cauliflower and fennel. Toss gently to coat. Pour into a baking dish.
Mix together the crumbs, shallots and melted butter. Top the cauliflower. Bake for 20 -30 min. until bubbly and top is golden brown. Don't let the shallots burn. Serve immediately.
Nutrition calculation is an estimation only. If you need nutritional calculations for medical reasons, please use a source that you trust.