Make the broth:.
Remove the meat from the crab legs. Usually, I'll break the legs at the joints, and use a pair of kitchen shears to cut them open.
Put the shells in the water with the carrots, chopped onions, and bay leaf. Bring to a simmer and simmer for 5-10 minutes. (If you have some extra crab, shrimp, or lobster shells in the freezer, you can throw some in.) If you don't have the carrots, you can leave them out - sometimes I add a wee bit of celery or a few whole peppercorns.
Strain the broth and put it back in the pan, on low heat.
Make the risotto:.
Heat up a small skillet on medium heat, add the oil, and onions. Soften then onions. Add the rice and stir till it’s a little toasted/opaque. Add the tomato paste and allow it to cook a little. Before it burns, add the wine, deglaze, let it simmer a little. Add enough broth to cover rice. Simmer, stirring or shaking the pan as necessary (probably every 30-60 seconds, try to keep the rice from sticking to the bottom), topping off broth as necessary. After 15 minutes or so, it should be close to done, taste the rice to check the texture. (I cook it al dente - a little bit of texture left, if you prefer mushier, go for it.) Season to taste with salt and pepper.
Add the crab meat. Cook for another minute to warm the meat. Remove from heat, add the butter and parsley, and stir to melt. Go easy on the parsley, but don't leave it out. If the rice is a little drier than you like, add a little more broth, but check the seasoning afterwards.
If you run low on broth, you can top it off with water. If you have a lot left over, use less water next time.