Preheat oven to 350 F.
In a large non stick saucepan, melt margarine. Add cellery and scallions; cook, stirring as needed, until softened, about 4 minutes.
Add the flour and cook, stirring constantly, until lightly browned, about 1 minute.
Stir in the milk and cook, stirring constantly, until the mixture boils and thickens, about 3 minutes.
Add the cheese, stir until melted.
Stir in the chicken, macaroni and peas; cook, stirring as needed until heated throughand the peas are thawed, about 3 minutes.
Transfer to a 1 1/2-quart casserole; top with cracker crumbs.
Bake until the crumbs are lightly browned, about 10 minutes.