Melt butter in a medium saucepan over medium-low heat. Chop onion and mince garlic; add to melted butter. Cover and allow to cook for 5 minutes.
Increase heat to medium-high and add wine; bring to boil and reduce liquid by half.
Remove skins and seeds from tomatoes, then dice and add to reduced liquid. Cook for 25 minutes, stirring occasionally to prevent sticking. Meanwhile, chop cilantro and shred chicken; set aside.
Remove tomato mixture from heat; allow to cool. Add to blender or food processor and puree. Return to saucepan and add cilantro, cayenne, and chicken. Bring to a simmer over medium heat. Season with salt and pepper and stir to combine; cook for 3-4 minutes. Serve topped with tortilla chips and avocado with grapes on the side.