Preheat oven to 350 degrees F. Set out a rimmed baking sheet.

Remove stems from mushroom caps (you can pull them right off). Set stems aside. Arrange mushroom caps top down on baking sheet. Brush mushrooms on all sides with olive oil, then sprinkle with 1/2 teaspoon sea salt and pepper to taste. Set aside. Mince mushroom stems, and reserve.

Arrange bacon in a large skillet and set over medium heat. Cook on both sides until edges are crisp, 5 - 7 minutes total. Remove bacon to a paper towel-lined plate. Once cool, mince.

Pour off and reserve all but 1 tablespoon bacon grease, and then add panko and a small pinch sea salt to the hot skillet. Stirring often, toast until panko is golden and is very fragrant, 2 - 3 minutes. Scrape toasted panko into a bowl. In the same skillet, still over medium heat, add a tablespoon of reserved bacon fat. Add onions and a small pinch sea salt, and sauté until translucent, 5 - 7 minutes. Add vermouth and cook until it's evaporated. Fold in herbs, toasted breadcrumbs, and bacon; remove from heat. Fold in 1/4 cup shredded cheddar. Taste and add salt or pepper as needed.

Spoon about 2 teaspoons filling into each of the prepared mushroom caps. Top mushrooms with remaining cheddar.

Bake 25 - 30 minutes or until filling is fragrant, and cheese is melted and bubbling. Garnish with fresh herbs and black pepper, and serve.