Ingredients

• 1 (8–9") soft hoagie roll or small Italian loaf, split in half lengthwise
• 1 tablespoon unsalted butter
• 1 small onion, finely chopped
• 8 ounces white or crimini mushrooms, trimmed, finely chopped
• Kosher salt, freshly ground black pepper
• 1/4 cup ketchup
• 1 cup grated part-skim mozzarella
• 2 slices deli ham, torn in half
• Mayonnaise, thinly sliced chives, french fried onions (such as French's), drained canned corn kernels, and/or chopped pickles (for serving)

Instructions

1. Preheat oven to 375°F; line a rimmed baking sheet with foil. Place bread halves cut side up on sheet.
2. Melt butter in a medium skillet over medium heat. Cook onion, stirring, until translucent, 3–4 minutes. Add mushrooms and lightly season with salt and pepper; cook, stirring occasionally, until liquid is evaporated and mixture cooks down to a spreadable consistency, about 10 minutes more. Let cool.
3. Spread 2 Tbsp. ketchup on each piece of bread. Divide mushroom mixture between pieces. Sprinkle mozzarella over each, leaving a bit in reserve. Top cheese with ham, then sprinkle ham with remaining cheese. Bake zapiekanki until bread is golden and crisp and cheese is lightly browned and bubbly, 12–14 minutes. Top with mayonnaise and your choice of toppings.