Bring a pot of water to a boil for the pasta. Salt water and cook pasta to al dente, about 8 minutes, adding Romanesco or cauliflower the last 4 minutes of cooking time.
Meanwhile, in a medium skillet, heat olive oil, 2 turns of the pan. Melt in 1 tablespoon butter and when it foams, add shallots and garlic. Season with thyme, rosemary, salt and pepper, and cook, stirring occasionally, until light golden, about 20 minutes. Add stock and stir.
In a medium saucepot, melt remaining butter over medium heat. Add salt, pepper, nutmeg, dry mustard or ground red pepper, and stir. Whisk in flour, cook 1 minute then whisk in milk. Thicken to coat spoon over medium-high heat, 2-3 minutes. Stir in cheese in figure-8 motion and remove from heat.
Combine pasta and Romanesco or cauliflower with sauce and shallots, and dish into a casserole. Top with Parm.
Bake until bubbly and brown, 35-45 minutes.