I found a great recipe in Quick Six Fix by Stuart O'Keefe, and I changed virtually nothing except to add about 1/4 cup of cream at the end. It gave the soup a wonderful silky mouth feel.
This calls for 1 teaspoon of dried red pepper. My red pepper flakes are ANCIENT and have probably lost a lot of potency. This was nice and spicy but even hubby could eat it (everything "has a little heat to it" to him.) I would recommend starting with less and doing a taste test to see if you want more. And if you're serving little ones, you will definitely want to cut back.
1/4 cup olive oil
2 medium onions, diced
4 cloves garlic, chopped
1 teaspoon red pepper flakes
2 teaspoons salt
2 teaspoons freshly ground pepper (I only used a couple twists of the pepper mill)
2 28-ounce cans diced tomatoes (preferably San Marzana)
2 cups vegetable stock
1 tablespoon sugar
2 tablespoons butter
Heat oil over medium heat. Add onion, garlic, red pepper flakes, salt and pepper and cook, stirring, for about 5 to 7 minutes until onions wilt.
Add tomatoes, stock and sugar and simmer on low for 10 minutes.
Puree soup and butter in a blender, food processor or with an immersion blender.
If desired, add a swirl of heavy cream. Serve with torn basil leaves stirred in.
PS It was delicious!