In a large skillet, melt butter over medium-high heat. Add mushrooms, onion, sweet pepper, 1/2 teaspoon salt and 1/2 teaspoon pepper; cook and stir until tender, 6-8 minutes. Add garlic; cook 1 minute longer. Stir in flour until blended. Gradually whisk in broth. Bring to a boil, stirring constantly; cook and stir until thickened, 1-2 minutes.
Transfer to a 5-qt. slow cooker. Stir in cream and noodles. Cook, covered, on low until noodles are tender, 4-5 hours. Meanwhile, in a small bowl, combine tuna, lemon juice and remaining salt and pepper.
Remove insert from slow cooker. Stir cheese, tuna mixture and peas into noodle mixture. Let stand, uncovered, 20 minutes. Just before serving, sprinkle with potato chips.