Heat the stock in a medium saucepan and keep warm over medium heat.
Melt the butter and olive oil in a large pan. Add the onion and cook over medium-high heat until the onion is translucent, about 5 minutes. Stir in the garlic and cook 30 seconds or until fragrant.
Add the middlins and stir until the grains are coated. Add the wine and cook until the liquid evaporates.
Add a ladleful of stock at a time (about ½ cup of the warm stock), and cook until the liquid is absorbed. Continue the cooking process, adding a ladleful of hot stock at a time, until the rice is almost al dente and there's about a ladleful or two of stock left, about 15 minutes. Season with salt and pepper.
Add the corn and the remaining stock and cook until the rice is al dente (it will have a slight bite to it but not be crunchy).
Gently fold in the crab, tarragon, and a little lemon juice. Divide the risotto between shallow bowls and garnish with more tarragon. Serve with lemon wedges, if desired.