In a large pot, melt butter over medium heat. Add onion, celery, leek, and carrot, and sauté for 3-4 minutes. Add diced potato and cook for 3-4 additional minutes. Add garlic, sautéing for 1 minute, then stir in the flour, rosemary, and thyme, cooking for 1-2 minutes continuously stirring so the flour doesn’t burn.