Melt butter in a sauce pan. Add chopped vegetables and garlic. Saute' over med-high heat 5 minutes or until tender. Add flour, cook stirring constantly, 1 minute or until smooth.
Add diced tomatoes and next 3 ingredients to slow cooker. Add basil, thyme, oregano, salt black pepper and red pepper to slow cooker. (If using fresh parsley add it 10 minutes before serving. If using dried parsley, add now.)
Stir and turn slow cooker to low. Cook on low 4 hours. Add crawfish and shrimp and cook 30 minutes to 1 hour depending on temperature of your slow cooker. Check at 30 minutes. If shrimp are pink, it's ready.