Cook the pasta according to the package instructions. Drain and set aside.
Heat the butter and olive oil in a large pot over medium-high heat. Add the broccoli, scallion whites, garlic, asparagus and bell pepper. Season with the salt and pepper and cook, stirring, for 3 minutes. Splash in the wine and allow it to reduce for 1 minute. Add the milk and cream and continue to cook until warmed through. Stir in the peas.
Toss together the fontina, Cheddar and cornstarch in a bowl.
While stirring the sauce, sprinkle in the cheese mixture. Stir until the cheese is completely melted. Bring to a simmer and allow the sauce to thicken, about 1 minute. Add the pasta and stir until well combined. Taste and adjust the seasoning to taste. Garnish with the reserved scallion greens.