Make-Ahead French Spaghetti - cooking recipe

Ingredients

1 medium onion, chopped
1/2 c. and 3 Tbsp. margarine, divided
1 (28 oz.) can peeled whole tomatoes, drained
salt
1 (16 oz.) pkg. spaghetti
2 Tbsp. flour
1 c. milk
1/2 pt. (1 c.) whipping cream
1/2 (6 oz.) can black olives, cut lengthwise
1 (4 oz.) can sliced mushrooms, drained
3/4 c. grated Parmesan cheese
1 tsp. salt

Preparation

Fry onion in 1/2 cup margarine until translucent.
Add tomatoes, stirring to break up.
Add 1 teaspoon salt.
Simmer 40 minutes, stirring occasionally.