In a small bowl, whisk together mayonnaise, Dijon mustard, honey, lemon juice, salt, and black pepper until smooth and slightly glossy. Taste and adjust sweetness or acidity by adding more honey or lemon as desired. Set aside.

Sandwich Assembly

Prepare bread

Lay out 8 slices of bread on a work surface. Spread a thin, even layer (about 1 tablespoon) of the honey Dijon spread on one side of each slice.

Build sandwiches

On four of the spread-topped slices, layer the ingredients in this order for even structure and flavor: 1) 1–2 slices of fontina cheese, 2) about 1/2 cup sliced chicken breast, 3) 2 slices smoked ham (folded if large), 4) 2 slices crisp bacon, 5) a few thin avocado slices (about 1/4 of the avocado per sandwich), 6) optional tomato slice and 1 leaf of lettuce if using. Top each assembled stack with another slice of fontina and then close with the remaining bread slice, spread side down.

Butter the bread

Spread about 1/2 tablespoon of room-temperature butter on the outer face of each sandwich (both top and bottom) to promote even browning when grilling.

Grill the sandwiches

Heat a large skillet or griddle over medium heat. When hot, place the sandwiches in the pan (work in batches if necessary). Press gently with a spatula or a heavy skillet to compress slightly. Cook 3–4 minutes on the first side, until golden brown and the fontina begins to melt. Flip carefully and cook another 2–4 minutes until the second side is golden and the cheese is fully melted. If the bread browns too quickly before the cheese melts, reduce heat to medium-low and cover the pan for 1–2 minutes to trap heat and encourage melting.

Rest and slice

Transfer sandwiches to a cutting board and let rest 1–2 minutes to let fillings settle. Slice each sandwich in half diagonally with a sharp knife for neat presentation.

Serve

Arrange sandwiches on plates with optional pickle slices. Serve immediately while hot and melty. Leftover spread can be refrigerated up to 3 days.