Butter an 8 x 12-inch gratin or casserole dish.

Toss Brussel sprouts in olive oil, salt and pepper, roast at 425°F for 25 minutes or until tender. Tossing one more time half way through.

Place bacon grease in pot, add butter and melt on medium heat. Add the leek and gently cook until softened, about 4 to 5 minutes.

Add the flour or cornstarch (if you use cornstarch, whisk with 1:1 cold water before adding) to the butter leek mixture and whisk for 1 to 2 minutes. Slowly add the cream while stirring with a wooden spoon until the roux is incorporated into the heavy cream. Cook over medium low heat for several minutes, stirring until the sauce thickens and coats the back of your spoon. Add fresh thyme.

Add 3 ounces of grated Gruyere cheese into the white sauce and stir until the cheese is melted and incorporated into the sauce. Add half of the cooked bacon back to the sauce. Taste and add more salt, if necessary.

Toast the panko in frying pan with 1 tbsp butter. Combine with the remaining bacon and 2 ounces of Gruyere. Set aside.

Add the Brussels sprouts into the cheese sauce and toss to coat. Transfer the cheesy Brussels sprouts into the prepared casserole or gratin dish. Sprinkle with the panko, Gruyere, and bacon mixture. Bake for 25 minutes until top is lightly browned and bubbly. Remove from oven and let sit for 10 minutes before serving.