Heat broiler to high and arrange a rack about 5 inches away from the heat.
Melt butter in a medium saucepan over medium heat. Add onions and season with salt and pepper. Cook, stirring occasionally, until golden and soft, about 10 minutes.
Add cheese, milk, and beer and cook until cheese is melted. Add pickle juice and cumin and stir until smooth. Remove from heat, add season with salt and pepper as needed. Cover to keep warm.
Slice cornbread into thick slices and place on a foil-lined baking sheet. Toast cornbread under the broiler for 2-3 minutes. Flip and cover each slice with cheese sauce. Return to the broiler to until cheese is golden, about 2 minutes more.
Top with pickled jalapeño slices, tomatoes, and cilantro. Serve immediately.