Step 1: Prepare the mint and pistachios. Wash and thoroughly dry the fresh mint leaves. Lightly toast the shelled pistachios in a dry pan over medium heat for 3-5 minutes until fragrant. Let them cool slightly.
Step 2: Make the pesto. In a food processor, combine the toasted pistachios, fresh mint leaves, garlic cloves, and 1/2 teaspoon of salt. Pulse several times until the mixture is finely chopped.
Step 3: Emulsify the pesto. With the food processor running, slowly drizzle in the 1/2 cup of olive oil until the pesto reaches your desired consistency. Stir in the grated Parmesan cheese and pulse a few more times to combine. Taste and adjust salt and pepper as needed.
Step 4: Cook the pasta. Bring a large pot of generously salted water to a rolling boil. Add the pasta and cook according to package directions until al dente. Before draining, reserve about 1 cup of the starchy pasta water.
Step 5: Grill the halloumi. While the pasta cooks, heat a non-stick skillet or grill pan over medium-high heat. Add a tiny splash of olive oil if needed. Place the halloumi slices in the hot pan and cook for 2-3 minutes per side, or until beautifully golden brown and slightly crispy.
Step 6: Combine pasta and pesto. Drain the cooked pasta and transfer it back to the large pot or a spacious serving bowl. Add the freshly made mint and pistachio pesto. Toss well to coat the pasta evenly.
Step 7: Adjust consistency and serve. If the sauce seems too thick, add a tablespoon or two of the reserved pasta water at a time, stirring until it reaches a silky, creamy consistency. Gently fold in the grilled halloumi slices. Serve immediately, garnished with extra fresh mint leaves and a sprinkle of Parmesan cheese, if desired.