Broccoli Rice Casserole

This recipe is one of my base recipes.  I regularly replace the broccoli with another dense vegetable such as winter squash, cauliflower, mushrooms and bell peppers.  Literally what ever vegetables I have available.
 1 1/2 cup rice
3  1/2 water
1 medium chopped onion
1 can (10 3/4 oz) condensed cream of  mushroom, or chicken, or celery or
cheese soup
1 1/2 cups 1% milk
2 –  10 oz packages frozen chopped broccoli or cauliflower or mixed 
vegetables
1/2 lb grated or sliced cheese
3 Tbsp margarine or  butter
Preheat oven to 350 degrees and grease 12x9x2x inch baking  pan.
In a saucepan mix rice, salt, and 3 cups of water and bring to a  boil.
Cover and simmer for 15 minutes. Remove saucepan from heat and set  aside
for additional 15 minutes.
Sauté onions in margarine or butter  until tender.
Mix soup, milk, 1/2 cup of water, onions, and rice. Spoon  mixture into
baking pan.
Thaw and drain the vegetables and then  spread over the rice mixture.
Spread the cheese evenly over the top and bake  at 350 degrees for 25-30
minutes until cheese is melted and rice is  bubbly.