Pour chicken or vegetable broth and clam juice into a medium saucepan and bring to a simmer. Lower heat and keep broth warm.
Optional: Remove ¼ cup of the warmed broth to a small heat-proof bowl and add saffron threads. Cover to keep warm and set aside.
In a separate larger saucepan, heat 1 tablespoon butter and olive oil over medium heat. Add onion, fennel, and shallot and cook about 5 minutes until starting to soften and translucent. Add garlic and cook for another minute, until fragrant.
Add arborio rice to the saucepan and cook for about 2 minutes, stirring a bit, until it’s translucent around the edges.
Pour wine (if using) into saucepan and cook, stirring until it's absorbed, about 2 minutes.
Add 1 cup of the warmed broth to the rice. Stir and let the rice absorb the stock. Once the broth is absorbed, continue to add more, a cup at a time, stirring and letting it absorb before adding more. You don't have to stir non-stop, but stir occasionally to make sure rice doesn't start sticking to the bottom of your saucepan. Continue with the rest of the broth, or until the rice is nice and tender, but still firm (I usually use all 4 ½ cups). Add saffron broth (you can keep threads in), too.
When rice is fully cooked, stir in remaining butter, lemon zest and juice, parmesan, and pepper. Taste and add in salt as desired. Remove saucepan from heat and gently fold in crab meat (you can reserve some for garnish).
Serve crab risotto in bowls and top with additional lump crab meat, chopped fennel fronds, and a lemon wedge if desired.