Make the chipotle aioli: in a small bowl, combine mayonnaise, pressed garlic, lemon juice, chipotle powder, salt, and pepper; stir until smooth and well combined.
Slice the baguette lengthwise and spread the aioli evenly over both cut sides.
Layer the bottom half with the turkey (or chicken) deli meat, then add the thinly sliced red onion, provolone slices, and avocado slices.
Close the sandwich and spread a thin layer of butter on the outside of both bread sides.
Cook the sandwich over medium-high heat in a preheated panini press until the bread is golden and crispy and the cheese has melted, or cook in a skillet using a heavy pan to press down the sandwich, about 3–5 minutes total depending on heat.
Remove from heat, slice into portions, and serve immediately.