Bring a large pot of salted water to boil. Add the spaetzle and cook according to the packet instructions until tender (typically 8-10 minutes for fresh). Drain and set aside.
Clean the mushrooms with a damp cloth and cut in half. Cut the pickled cucumber into half slices. Wash the cherry tomatoes and cut in half. Peel and dice the onion. Cut the chicken breast fillets into bite-sized pieces.
Heat a little oil in a large skillet over medium-high heat. Working in batches to avoid crowding, fry the chicken pieces for 3-4 minutes per batch until golden brown. They do not need to be cooked through yet. Transfer to a plate and set aside.
In the same skillet, sauté the diced onion over medium heat for 2-3 minutes until soft and translucent.
Add the mushrooms and pickled cucumber to the skillet and fry over medium-high heat for 3-4 minutes, stirring occasionally, until the mushrooms are golden.
Pour in the light beer and chicken broth, scraping the bottom of the skillet with a wooden spoon to deglaze. Simmer for 1 minute.
Stir in the cream and bring to a boil over medium heat, about 2 minutes. Add the grainy mustard and stir well. Season with salt and pepper.
In a small bowl, mix the cornstarch with 2 tablespoons of cold water until smooth. Slowly pour this mixture into the sauce while stirring constantly, and simmer for 2-3 minutes until thickened.
Return the browned chicken and cherry tomatoes to the skillet. Simmer gently for 3-4 minutes, stirring occasionally, until the chicken is heated through and reaches an internal temperature of 74°C (165°F).
Drain the cooked spaetzle and toss gently with the warm butter until coated evenly. Season with salt and a pinch of nutmeg.
Divide the spaetzle among serving plates and spoon the chicken and mushroom sauce over top. Serve with a fresh green salad on the side.
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