Preheat oven to 350°F. Bring a pot of water to a boil for the pasta.
Heat large skillet with olive oil, 2 turns of the pan, over medium to medium-high heat. Add vegetables and scapes or garlic, and season with salt and pepper. Sauté to tender-crisp, 5 minutes or so. Turn off heat, douse with lemon juice and add tarragon.
In a medium, high-sided skillet or pot, melt butter over medium heat. Add flour and whisk 1 minute; whisk in chicken stock and thicken 1-2 minutes. Slowly whisk in warm milk and season sauce with salt, pepper and nutmeg to taste. Thicken to lightly coat the back of a spoon. Stir in Fontina or Gruyère and Asiago in a figure-8 motion, a handful at a time, using a wooden spoon.
Undercook pasta by 2-3 minutes in salted boiling water, 6-7 minutes depending on package directions. Drain the pasta and add back to pot with cheese sauce and vegetables. Stir to combine and adjust salt and pepper to taste. Transfer to casserole and cover with Parm. Bake to brown and bubbly, 20-25 minutes.