1. Preheat oven to 400 degrees Fahrenheit.
2. Place blintzes in a 9×13 pan.
3. In a separate baking dish, toss broccoli with oil and salt and roast for 20 minutes.
4. Heat butter in a small pot until melted, then sauté onion until translucent. Add flour, milk, cheese, and salt to taste. Stir until cheese is fully melted and sauce is smooth.
5. Reduce oven temperature to 375 degrees Fahrenheit.
6. Top blintzes with roasted broccoli, then pour sauce over it.
7. Cover and cook for 25 minutes.