Bring a large pot of well-salted water to a boil. Cook the penne according to package directions for al dente. Reserve at least 1/2 cup of pasta cooking water before draining. Drain but do not rinse the pasta.

While the pasta cooks, toss the cubed chicken with Italian seasoning, salt, and pepper. Heat olive oil in a large skillet over medium-high heat until shimmering. Add the chicken in a single layer — cook in two batches if needed to avoid crowding. Cook for 5 to 6 minutes, stirring occasionally, until golden and almost cooked through.

Add the halved cherry tomatoes to the skillet. Cook for 2 to 3 more minutes until tomatoes soften and chicken is fully cooked through. Stir in the fresh spinach and cook until just wilted, about 30 to 60 seconds. Remove from heat.

Add the drained pasta, 2 tablespoons of reserved pasta water, and the pesto to the pan. Toss until evenly coated. Add more pasta water one tablespoon at a time if the sauce is too thick.

Top with toasted pine nuts, fresh basil, and red pepper flakes if using. Serve immediately.