Cut the onions, apples, and camembert into 3 mm thick slices. Apples do not need to be peeled.
Halve the chicken breast fillets lengthwise and season with salt and pepper.
Heat a non-stick pan over medium heat and add some butter. Sauté the onion slices until golden brown, about 5-8 minutes, turning several times. Season with salt and pepper and transfer to a plate.
Add more butter to the pan over medium heat. Fry the apple slices for about 1 minute on each side, sprinkle with thyme, and season with salt to taste. Transfer to a plate.
Halve the pretzel rolls. Add a small amount of butter to the pan over medium heat, place roll halves cut-side down, and toast for 2-3 minutes until crispy. Place on an oven rack.
Add some olive oil to the pan and heat over medium-high. Sear the chicken fillets on both sides until golden, about 2-3 minutes. Season with salt and pepper. Continue cooking over medium heat, turning occasionally, for 5-7 minutes until the internal temperature reaches 74°C (165°F). Transfer to a plate.
Mix the mustard with the honey.
Brush the bottom halves of the toasted rolls with the honey-mustard sauce. Layer with some onion slices and 1-2 apple slices. Place a chicken fillet on top, then add 2 camembert slices, an apple slice, and more onion slices. Cover with the top roll half.
Place the burgers in the preheated 180°C (356°F) oven for 2 minutes to melt the cheese, then serve.
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