Preheat oven to 425°F (220°C).
Place the whole red peppers on a baking sheet and roast for 30-40 minutes, until blackened and blistered.
Remove from the oven and place the peppers in a paper bag to seal. Let them steam for 15 minutes.
Once cool enough to handle, peel off the skin and remove the seeds and stems.
In a large saucepan or Dutch oven, heat olive oil over medium heat. Add the onion and cook until softened, about 5 minutes.
Add the garlic, smoked paprika, and red pepper flakes (if using) and cook for another minute until fragrant.
Add the diced tomatoes, vegetable broth, and roasted red peppers. Bring to a boil, then reduce heat and simmer for 20 minutes, until thickened.
Add the cashews and fresh basil leaves to the pot and blend using an immersion blender or transfer to a blender and blend until smooth.
Season with salt and pepper to taste.