Preheat oven to 350°. Spray a 13x9 pan with nonstick cooking spray.
Add broccoli florets to a large heavy-bottomed stockpot of boiling water. Cook for 3-5 minutes and then drain broccoli from the water. Set aside.
Melt butter over medium heat in a small sauce pan. Whisk in flour until well-combined, then whisk in milk, salt, pepper, cayenne pepper and 1 cup of Monterrey Jack cheese. Reduce heat to simmer and continue to whisk until sauce simmers slightly around the edges and has thickened slightly. Stir in sour cream.
Arrange broccoli in casserole dish and then pour cream sauce over broccoli. Top with cheddar cheese and crushed crackers.
Bake uncovered for 35-40 minutes.