Cauliflower can retain moisture and you want it moisture free when making a gratin.
There are 2 alternatives for cooking in this recipe.
1 large head cauliflower (6 cups) - See Notes
1/4 c. melted butter, divided
1/2 c. diced onion
1 c. sour cream
1-1/2 c. shredded American cheese
1/4 t. salt
1/2 c. dried breadcrumbs
Break cauliflower into florets. Cook for 15 minutes in boiling salt water; drain well. Or roast in oven (see Notes)
Combine cauliflower with 2 Tbls. melted butter and onion, sour cream, cheese and salt. Spoon into a 1-1/2 qt. casserole dish.
Melt remaining melted butter and toss with bread crumbs. Sprinkle over the cauliflower mixture.
Bake at 350 degrees for 30 minutes, or until heated through.