In a large pot, bring salted water to a boil. Add the cauliflower florets and cook for about 5 minutes, until slightly tender. Drain and set aside.
In a large skillet, heat the butter and olive oil over medium heat. Add the chopped onion and sauté for 3-4 minutes, or until translucent. Stir in the minced garlic and cook for an additional minute, until fragrant.
Reduce the heat to low and pour in the heavy cream. Whisk in the Dijon mustard, salt, black pepper, and nutmeg. Gradually add the shredded cheddar and Gruyère cheese, stirring until melted and smooth.
In a large mixing bowl, combine the drained cauliflower florets with the cheesy sauce. Gently fold until the cauliflower is well-coated.
Transfer the cheesy cauliflower mixture to a greased baking dish. Sprinkle the breadcrumbs evenly over the top to create a crunchy crust. For added flavor, sprinkle paprika or cayenne on top if desired.
Place the baking dish in the preheated oven and bake for 25-30 minutes, or until the top is golden brown and bubbly.
Once out of the oven, let it cool for a few minutes. Garnish with freshly chopped parsley before serving.
Scoop portions onto plates and serve warm as a delightful side dish at your festive gatherings.
Notes
For extra flavor, consider adding paprika or cayenne.