Crock Pot Creamy Tomato Soup

In a skillet over low/medium heat, melt the butter and sauté the finely diced onions and garlic until softened.
Sprinkle the flour over the softened, butter-covered onions, stirring until coated. Cook for an additional minute, then turn off the heat.
Transfer the onion roux mixture to the slow cooker.
Add the can of diced tomatoes, chicken broth, salt, oregano to to the slow cooker.
Gently stir everything to combine, cover with the lid, and set the slow cooker to high for 2 hours. If cooking on low, set it to 4 hours.
Stir the soup halfway through the cooking time.
Use an immersion blender to blend the soup until smooth directly in the slow cooker. Alternatively, transfer the contents to a blender and puree until smooth.
Pour the blended mixture back into the slow cooker and set it to low.
Stir in the half and half, ensuring it's well combined.
Cover with the lid and let it cook on low for an additional 30 minutes. Add the basil leaves in the end and serve.