For the crab stock:
For the petite croutons:
Bring crab stock to a simmer in a medium pot. Heat the olive oil in a wide shallow saucepan over medium-high heat. Add onion, fennel, and leek and cook until soft, but not browned, four to five minutes. Add garlic and cook until fragrant, about one minute. Stir in rice and cook, stirring frequently, until slightly toasted, two to three minutes. Add wine and stir frequently until fully absorbed, one to two minutes. Add the stock, half a cup at a time, stirring frequently and waiting until liquid is absorbed before adding more. Continue this process for approximately 15 minutes. When rice is al dente (firm, not hard), add two tablespoons of the cream and stir until almost completely absorbed, about two minutes. Fold in the cold butter and a fourth of the crab meat.
Put the remaining two tablespoons of cream and chile powder in a small pan over medium-high heat and stir until the chile powder is incorporated. Add the remaining crab meat, chervil leaves, and 10 tarragon leaves reserved from the crab stock. Spoon the finished risotto onto a serving plate and spoon the chile cream over it. Garnish with a few fresh tarragon leaves reserved from crab stock. Sprinkle with the petite croutons.
For Crab Stock:
Preheat oven to 400ºF.
Spread crab shells on baking sheet and roast for 20 minutes.
Heat the canola oil in a large pot over medium-high heat. Add the onion, celery, carrot, leek, and garlic and cook, stirring often, until soft but not brown, approximately three to four minutes. Add roasted crab shells, bay leaf, thyme, tarragon stems, lemon, peppercorns, and water. Increase to high heat and bring to a boil. Immediately reduce heat to low and gently simmer for 1½ hours, skimming off any scum that may come to the top. Strain through a fine sieve, discard solids, and allow to cool.
For Petite Croutons:
Heat the olive oil in a sauté pan over medium-high heat. Add the garlic, thyme, and rosemary. Add the bread and toast it, stirring frequently, until golden brown. It will continue to brown when removed from the oil, so be careful not to overcook. Drain on paper towels.