Whisk together the butter, mustard, garlic, 1 teaspoon salt and several grinds black pepper in a large microwave-safe bowl. Add the green beans, tomatoes and peppers and toss to coat. Cover with plastic wrap and microwave until the green beans are crisp-tender and still bright green, about 2 minutes. Remove the plastic wrap and let cool completely.
Transfer the vegetables with the marinade to a large resealable plastic bag and add the chicken. Squeeze out most of the air and seal, then toss gently to coat the chicken with the marinade. Freeze until ready to use. Let thaw in the refrigerator for at least 24 hours before cooking
Pour the chicken, vegetables and marinade into a large skillet and heat over high heat. Cook, stirring often, until the chicken is cooked through, about 12 minutes. Season with salt and pepper.
Divide the chicken and vegetables among 4 plates. Sprinkle with the parsley and Parmesan and serve with a piece of baguette.
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