1 head cauliflower (about 2-1/2 pounds, trimmed and cut into florets)
1/4 cup olive oil ( divided)
  dash Kosher salt
  dash Freshly ground black pepper
2 tablespoons unsalted butter (plus extra for the baking dish)
1/2 cup panko (Japanese breadcrumbs)
2 tablespoons Parmesan cheese (finely grated)
1/4 cup fresh flat leaf parsley (minced, divided)
2/3 cup yellow onion (finely diced)
1 large clove of garlic (finely minced)
2 1/2 tablespoons unbleached all-purpose flour
1 1/2 cups lowfat (2%) or whole milk
2 cups Gruyère cheese (8 ounces) (shredded)