Crabs: Preheat oven to 425 F. Place crabs on a roasting pan and bake in the oven about 30 minutes, until deep red and slightly browned. In a stockpot set over medium heat, add olive oil and roasted crabs, breaking them up with a wooden spoon as they cook, 5 minutes. Add onion, bell pepper, and celery (the "holy trinity" of Cajun cooking) and stir, cooking just till the vegetables soften, 5 minutes.
Stock: Add wine and stir to deglaze the pan. Turn heat to high and cook 6–8 minutes, until the wine has nearly evaporated. Add tomato paste and water. Stir, bring to a boil, then reduce to low heat. Cover and simmer, 45 minutes.
Bisque: Remove the crab shells from the stock and discard. (If you miss a couple small pieces of shell, don't worry: they'll get strained out later.) Ladle the stock into the blender, then add the juice of half a lemon and ¾ cup bread cubes. Season with a big pinch of salt and a small pinch of pepper. Cover and let sit so the bread can absorb some of the liquid, 30–60 seconds. Blend on low speed (to avoid splattering hot liquid) for 30 seconds. As the soup blends, turn to high to emulsify, 1 more minute. Taste to adjust for seasoning and texture—the soup should coat the back of a spoon. Add remaining bread, small pinches of salt and pepper, and blend on high to further thicken and emulsify, 1½ more minutes. Taste again for seasoning, then strain through a chinois to remove any small shell pieces, using the back of a ladle to push the soup through into a bowl.
Assembly: Ladle bisque into individual serving bowls. Chop chives into ½-inch lengths (you should have about a ½ cup total). Garnish each bowl with crab meat and chives. Serve immediately.