Preheat oven and roast the spaghetti squash until tender, then let cool slightly and shred the flesh into strands.
Make the pesto: add basil, pumpkin seeds, and garlic to a food processor and pulse to chop.
With the processor running, slowly stream in 2/3 cup avocado oil until the pesto reaches your desired texture; transfer to a bowl and stir in Parmesan if using.
Heat 1 to 2 tablespoons avocado oil in a skillet over medium-high heat.
Season the chopped chicken thighs with paprika, garlic powder, and 1/4 teaspoon sea salt.
Add the chicken to the hot skillet and brown without stirring for 3 minutes, then stir and brown without touching for another 3 minutes.
Stir the chicken, cover the skillet, and cook until the chicken is cooked through, about 2 to 5 more minutes; remove from heat.
Combine the shredded spaghetti squash strands and the pesto in a large mixing bowl and toss to coat evenly.
Add the cooked chicken, including any pan juices, to the bowl and toss until everything is well combined; adjust sea salt to taste.