Preheat the oven to 400°F (200°C). Line a rimmed baking sheet with foil and place a wire rack on the sheet if available. Arrange the 8 slices of applewood-smoked bacon in a single layer on the rack or directly on the foil. Bake 12–16 minutes until crisp to your liking. Remove and transfer to a paper-towel-lined plate to drain and keep warm.

Prepare vegetables

While bacon bakes, slice the tomato into 1/4-inch (6 mm) thick rounds. Peel and thinly slice the 1/2 small red onion into rings. Place tomato slices on a plate and lightly season both sides with about 1/4 teaspoon of the salt and a pinch of black pepper; set aside.

Toast croissants and make spread

Slice croissants in half horizontally. In a small bowl, mix the softened butter, Dijon mustard and mayonnaise until smooth. Lightly brush the cut sides of each croissant with the butter-mustard mixture. Heat a large skillet or griddle over medium heat. Add 1 teaspoon olive oil and place croissant halves cut-side down in the pan in batches if necessary. Toast 1–2 minutes until lightly golden and warmed; remove and keep warm.

Assemble sandwiches

Lay the bottom halves of the toasted croissants on a sheet pan or ovenproof tray. On each bottom half place 2 slices of Black Forest ham (folded if needed), then divide the baked bacon (2 slices per croissant). Top meat with about 1/4 cup shredded or a slice of sharp cheddar cheese (approx. 1/4 cup shredded or 1 slice). Add one or two tomato slices and a few thin red onion rings on top of the cheese.

Melt cheese and finish

Place the assembled croissants (open-faced) under a broiler on high, about 4–6 inches from the element, for 1–2 minutes until the cheese is melted and just beginning to bubble. Watch closely to avoid burning. Alternatively, preheat oven to 375°F (190°C) and bake 4–6 minutes until cheese melts.

Serve

Remove from broiler/oven. Season tomato/onion with remaining salt and a pinch of black pepper if desired. Place croissant tops on sandwiches and serve immediately while warm.