Preheat oven to 400°F (200°C). Butter a shallow 10- to 12-cup (2.5 to 3 L) baking dish.

Sprinkle chicken breasts with 1/4 tsp (1 mL) each salt and pepper. In a large nonstick skillet, melt butter over medium heat; add chicken and cook, turning once, for about 10 min or until chicken is well browned; transfer to a plate.

Add green onions to skillet; sauté for 1 min. Whisk flour into milk; gradually pour into pan. Cook, stirring constantly; for about 3 min or until slightly thickened. Remove from heat, stir in basil, 1/2 cup (125 mL) of the cheese and remaining salt and pepper.

In a large pot of boiling salted water, cook noodles for about 5 min or until almost tender, adding green beans 2 min before end of cooking (or 1 min if frozen); drain and spread in baking dish.

Pour 3/4 of sauce over noodles in dish; toss gently to coat. Arrange chicken and tomatoes on top. Spoon remaining sauce on chicken; sprinkle with remaining cheese. Bake for about 15 min or until thermometer inserted in thickest part of chicken registers 165°F (74°C) and is no longer pink inside.

Replace green beans with broccolini, blanching as directed, or with chopped drained marinated artichoke hearts and add them with tomatoes. Add 1 cured chorizo sausage, sliced, with tomatoes.