Place a 4-inch square of doubled cheesecloth on the counter. Place fennel seed, thyme, bay leaf and basil in the center. Bundle cheesecloth around mixture and tie into a little pouch with a piece of kitchen twine. Set aside.
Pour oil into a large, heavy skillet. Place over medium heat.
When oil is hot and shimmering, add prepared onion. Cook, stirring occasionally, until very soft but not colored, about 10 minutes.
Add red peppers and jalapeño. Continue to cook, without browning, until very tender, about 8 minutes more.
Stir in flour. Cook for 1 minute.
Add tomato paste. Cook for another minute.
Add a little of the hot chicken stock, stirring until smooth. Then add remaining chicken stock, plum tomatoes. and the prepared spice bag.
Adjust heat to bring the mixture to a gentle simmer.
Add teaspoon salt and several turns of the peppermill. Stir to mix.
Simmer uncovered, stirring occasionally, for about 30 minutes, until slightly thickened.
Remove from heat and discard the spice bag.
In batches, purée in blender until very smooth or use an immersion blender.
Press the soup through a fine sieve and return to a clean saucepan.
Stir in the cream. Return to a slow simmer.
Taste. Add more salt and pepper, to taste, if needed.
Simmer for about 1 minute more.
Ladle into warmed soup bowls and serve.