Bring a large pot of water to boil; cook pasta until al dente.
While pasta is cooking, in med saucepan, melt 2 T of the margarine. Remove from heat; add flour and stir until blended in. Whisk in stock and milk, stirring over med heat until mixture comes to a boil and thickens. Reduce heat to low. Stir in cheeses and seasonings. Continue stirring until cheese is melted. Remove from heat and set aside.
In a large skillet, melt remaining 1 T margarine. Add remaining ingredients. Cook vegetables, stirring constantly, for about 5 minutes, until tender/crisp. Reduce heat to low. When noodles are done, drain well. Toss with vegetables; stir in cheese sauce. Garnish with scallion curls.