In a large pot over medium-high heat, sautė the mushrooms in melted butter. Cook until tender and lightly browned, about 5 minutes.
Add the onion and cook until soft, 6-8 minutes.
Add the flour gradually and stir to mix over medium heat.
Pour 1 cup of chicken broth and whisk until flour dissolves. Add the rest of the chicken broth.
Stir in the milk and add salt, fresh ground black pepper, and onion powder. Simmer for 10 minutes.
Add the chicken, heavy cream, green peas, and roasted red peppers and stir to combine. Cook until heated through. Taste and adjust seasoning. Transfer to a serving plate.