Preheat your oven to 350°F. Grease a 2-quart ovenproof dish and place it on a baking sheet.

Heat the olive oil in a large skillet over medium heat. Add the chopped bacon and cook until crisp. Remove from the pan and set aside on a paper towel. In the same pan, add the sliced leeks and cook over medium-low heat for about 10 minutes until soft. Add the minced garlic and sauté for an additional 2 minutes. Transfer this mixture to the prepared ovenproof dish.

Bring a large pot of salted water to a boil and add the cauliflower florets. Cook for 5-7 minutes until slightly tender. Drain well and set aside.

In a medium saucepan, melt the butter over medium heat. Stir in the flour and cook for 1-2 minutes until it forms a paste (roux). Gradually whisk in the milk, ensuring no lumps form. Continue to stir until the sauce thickens and begins to bubble. Add the cheddar cheese, Dijon mustard, dried thyme, salt, and pepper. Stir until the cheese is fully melted and the sauce is smooth. Stir in the heavy cream.

Add the par-boiled cauliflower and cooked bacon to the cheese sauce. Gently toss to combine. Pour the cauliflower mixture over the leek and garlic layer in the ovenproof dish. Sprinkle the top with the grated Parmesan cheese.

Bake in the preheated oven for 25-30 minutes or until the top is golden brown and the gratin is bubbling.

Let the gratin cool for a few minutes before serving. Enjoy as a side dish!