Preheat the oven to 425F. Place the heavy cream, butter, garlic, mustard, salt, and pepper in a small saucepan. Place over medium heat and stir until the butter has melted and the garlic becomes fragrant, about 5 minutes. Set aside.

Slice the cauliflower into 1/2-inch to 1-inch slices. Thinly slice the yellow onion. In an 8 or 9 inch baking dish, evenly spread 1/3 of the cauliflower, then 1/3 of the onions, and 1/3 of the cheese. Continue to layer until all the cauliflower, onion, and cheese is used, finishing with cheese on top.

Carefully pour the infused cream over the layered cauliflower. Cover the dish with aluminum foil and bake for 45-55 minutes. Uncover and bake for 10-20 minutes, until the cheese is starting to brown and the cauliflower is fork tender.

In a small bowl, combine the panko, olive oil, and Italian seasoning. Evenly spread the panko mixture over the cauliflower. Return to the oven and bake another 10 minutes, until the panko is golden brown.

Top with chives and enjoy!