Marinate the Shrimp: Combine the buttermilk, shallot, garlic and lemon zest in a small bowl, mix in the shrimp, and set aside while you make the risotto. If you can prepare this earlier and let the shrimp marinate longer, even better.
Make the Risotto: Pour the broth into a large saucepan and bring to a boil. Reduce the heat to low for a gentle simmer.
Place a large heavy pot over medium heat and add the olive oil and 1 tbsp of butter. Once the butter has melted, add the chopped onion and celery and cook at medium heat, stirring frequently until soft but not brown, about 10 minutes.
Mix in the rice and cook for 1 minute longer. Pour the prosecco and stir continuously until it has evaporated. Reduce the heat to medium/low. Add the broth to the rice one cup at a time, stirring and waiting until broth is fully absorbed before adding the next cup. Continue adding the broth until the rice is soft, but still
toothsome, about 15 to 20 minutes longer.
Remove from heat. Beat in the remaining butter, the grated Parmigiano, lemon zest, lemon juice. Check the seasoning and add salt and pepper if needed. Put a lid on the pan and leave the risotto to rest for a minute.
Cook the Shrimp: Drain the shrimp, shaking off any excess marinade , and pat dry with paper towels. Heat a large skillet over medium/high heat and add 2 tblsp of the olive oil. Season the shrimp with salt and pepper and add them to pan. Cook, stirring occasionally, until the shrimp turn plump and begin to brown.
Add the peas and mint to the risotto, mixing quickly to incorporate well. Scoop the risotto onto a large serving dish or individual plates. Arrange shrimp on top, sprinkle with lemon zest and mint and serve.