Adjust rack to top position and preheat oven to 450 degrees. Wash and dry produce.

Heat a large dry pan over medium-high heat. Add bacon*; cook, turning occasionally and adjusting heat if browning too quickly, until crispy, 6-10 minutes. Turn off heat; transfer to a paper-towel-lined plate. Carefully discard all but 1 tsp bacon fat (2 tsp for 4 servings) from pan. (You’ll use this to cook the onion later.)

While bacon cooks, cut potatoes into ½-inch-thick wedges. Halve, peel, and thinly slice half the onion (whole onion for 4).

Toss potatoes on a baking sheet with a drizzle of oil, salt, and pepper. Roast on top rack until golden brown, 20-25 minutes.

Meanwhile, add a drizzle of oil to pan with reserved bacon fat; heat over medium-high heat. Add sliced onion; cook, stirring occasionally, until browned and softened, 8-10 minutes.

Add stock concentrate, half the jam (you’ll use the rest later), ¼ cup water (½ cup for 4 servings), and 1 tsp sugar (2 tsp for 4); cook, stirring occasionally, until onion is caramelized and jammy, 2-3 minutes more. Season with salt and pepper.

Turn off heat; transfer to a plate. Wash out pan.

Top half the sourdough slices with even layers of Monterey Jack and caramelized onion; drizzle with remaining jam. Arrange bacon and gouda on top. TIP: If gouda slices don’t fit, cut them in half and shingle on sandwiches.

Close sandwiches with remaining sourdough slices.

Melt 1 TBSP butter in pan used for onion over medium heat. Add sandwiches and cook, pushing them around pan to absorb butter, until bread is golden brown on one side and cheese begins to melt, 4-6 minutes.

Add another 1 TBSP butter to pan and flip; cook until bread is golden brown on second side and cheese melts. (For 4 servings, work in batches or use a second pan, adding 1 TBSP butter for each side.)

Halve melts on a diagonal.

Divide melts between plates and serve with potato wedges on the side.