Preheat oven to 400 degrees F.

In a small bowl, combine olive oil, Panko and 1/4 cup Parmesan; set aside.

Place cauliflower in a steamer basket and set over boiling water. Cover and steam until crisp-tender, about 4-5 minutes; drain well.

Melt butter in a large skillet over medium heat. Add garlic and shallots, and cook, stirring frequently, until fragrant, about 2 minutes.

Whisk in flour and thyme until lightly browned, about 1 minute. Gradually whisk in milk, heavy cream and Dijon. Cook, whisking constantly, until incorporated and slightly thickened, about 4-5 minutes. If the mixture is too thick, add more milk as needed; season with salt and pepper, to taste.

Stir in cauliflower, mozzarella and remaining 1/4 cup Parmesan until the cheeses have melted, about 1-2 minutes. Sprinkle with Panko mixture.

Place into oven and bake until golden brown and bubbly, about 20-25 minutes.

Serve immediately, garnished with thyme, if desired.