Bring a medium pot of salted water to a boil. Add broccoli florets and blanch for 2-3 minutes until bright green and slightly tender. Drain and pat dry with paper towels.
In a skillet over medium heat, melt the butter. Add diced onion and cook for 4-5 minutes until softened. Add garlic and cook 1 minute more. Remove from heat.
Grease a 9x13-inch baking dish. Spread the bread cubes evenly across the bottom.
Layer the blanched broccoli and sautéed onion mixture over the bread. Sprinkle with 2 cups of the shredded cheddar cheese.
In a large bowl, whisk together eggs, milk, heavy cream, salt, pepper, and nutmeg until well blended.
Pour the egg mixture evenly over the casserole, making sure to saturate all the bread cubes. Gently press down to ensure everything is moistened.
Top with the remaining 1 cup of cheddar cheese.
Cover with aluminum foil and refrigerate for at least 1 hour or overnight.
When ready to bake, preheat oven to 350°F. Bake covered for 35 minutes, then remove foil and bake 15 more minutes until golden and set in the center.
To Freeze: Complete through step 7, then wrap tightly with plastic wrap and foil. Freeze up to 3 months. Thaw in refrigerator overnight before baking as directed.