Bring a large pot of salted water to a boil and cook the whole grain penne according to package directions until al dente.
While the pasta cooks, heat the olive oil in a large skillet over medium-high heat.
Add the sliced bell peppers and diced onion to the skillet, sautéing for 3-4 minutes until they begin to soften.
Stir in the minced garlic and sauté for another 30 seconds until fragrant.
Add the shrimp to the skillet and season with Cajun seasoning, sea salt, and black pepper.
Cook the shrimp for 2-3 minutes per side until they turn pink and opaque.
Pour in the tomato puree and a splash of pasta water, stirring to coat the shrimp and vegetables in the sauce.
Drain the pasta and add it directly to the skillet, tossing everything together to combine.
Finish with a squeeze of fresh lemon juice and a garnish of chopped parsley before serving.