In a medium bowl whisk together the lemon juice, garlic and olive oil. Add the shrimp and toss well. Set aside while you prepare the other ingredients.
Boil the pasta in a large pot of boiling salted water. I use 2 tablespoons of salt in the pot.
Cook the pasta for the length of time on the package minus one minute. We want it to be quite al dente.
Rinse the cooked pasta under cold water, drain and set aside.
Cook the shrimp on a grill or in a hot frying pan until charred, about 2 minutes on each side.
In a large bowl mix together the pesto and ricotta. Add the pasta and mix well.
Add the onion, tomatoes and shrimp and mix gently.
Top with the crumbled feta (if using) and serve.