Bring a large pot of water to a boil and add in the Brussels sprouts and a sprinkle of sea salt.

Boil Brussels sprouts for about 10 minutes, until tender but not totally cooked.

Drain the Brussels sprouts and pat dry with a towel.

Preheat oven to 425 degrees F and line 2 baking sheets with parchment paper.

In a small bowl, mix together the olive oil, dijon mustard, salt, and pepper.

Toss the Brussels sprouts in a large bowl with the marinade.

Arrange the sprouts in a single layer on a baking sheet, then use the bottom of a small glass or mason jar to press down on the sprouts to smash them flat.

Sprinkle grated Parmesan cheese on top if using, then bake in the oven for about 25-30 minutes or until crispy/golden brown.