Heat Broiler. On a baking sheet lined with aluminum foil, toss broccoli with oil. Season the brocolli with sea salt and ground pepper. Broil, tossing every 2 min, for 4 to 6 min; broccoli should just begin to char. Add the mushrooms and bell peppers and season with more sea salt and ground pepper. Toss to combine the veggies. Broil, tossing one or two times, for 8-10 min. Meanwhile, in a small bowl, combine mayonnaise and garlic then season with sea salt and ground pepper. On a work surface lay out the bread slices and spread the mayonnaise mixture onto each slice. Now take the broiled veggies and top each slice. Next take the Gouda and top each piece. Place the slices on a baking sheet and broil until the cheese melts and starts to lightly brown, about 2-4 min. Serve warm.