Cook buckwheat according to the instructions on the package. This will take about 15-20 minutes.
Meanwhile, heat cooking oil over medium heat. Saute onion for 2-3 minutes. Then add garlic and Tex Mex seasoning. Cook for 2-3 minutes more. Stir regularly.
Add bell pepper and eggplant, cook for 2-3 minutes more, stir regularly. Add more oil if needed.
Stir in tomatoes and water. Season with salt and pepper to taste. Cover and cook for 10-15 minutes, or until tomatoes start to fall apart.
Stir in kidney beans and cook until heated through.
Meanwhile, add all avocado sauce ingredients in a blender and process until smooth. Add water if needed to improve consistency. Set aside in the fridge.
When ready, serve with cooked buckwheat and avocado sauce.