Melt butter in a large Dutch oven over medium heat. Add onion and cook for 4–5 minutes until softened. Add garlic and cook for 30 seconds until fragrant.
Sprinkle in flour and stir constantly for about 1 minute until it forms a paste.
Slowly whisk in broth until smooth and no lumps remain.
Add milk, heavy cream, thyme, poultry seasoning, garlic powder, onion powder, salt, and pepper. Bring to a gentle simmer.
Cook for 5–7 minutes, stirring occasionally, until the mixture is slightly thickened.
Add frozen vegetables and shredded rotisserie chicken. Simmer for another 5–7 minutes until vegetables are tender and everything is hot.
Taste and adjust salt and pepper as needed.