Did a search and found this. Hope this is the one. I hate it when I lose a recipe.

Ingredients

2 tbsp (30 ml) cornstarch
1 tsp (5 ml) salt
1/2 tsp (2 ml) dry mustard (optional)
1/4 tsp (1 ml) pepper
2-1/2 cups (600 ml) milk
2 tbsp (30 ml) Margarine or butter
2 cups (475 ml) (8 oz (224 grm).) shredded American or Cheddar cheese, divided
8 oz (224 grm). Mueller's Elbows (about 2 cups), cooked 5 minutes and drained

Preparation
In medium saucepan combine corn starch, salt, dry mustard and pepper; stir in milk.
Add margarine.

Stirring constantly, bring to a boil over medium-high heat and boil 1 minute.

Remove from heat.
Stir in 1-3/4 cups (425 ml) cheese until melted.
Add elbows.

Pour into greased 2-quart casserole.
Sprinkle with reserved cheese.

Bake uncovered in 375 degree (200 C.) oven 25 minutes or until lightly browned.

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