Make the meatballs: Combine the ground turkey, ground beef, and Italian seasoning in a large bowl. Mix gently just until combined — overmixing makes meatballs dense and tough. Form into evenly sized meatballs and place on a baking sheet. If the mixture feels sticky, lightly oil your hands rather than adding breadcrumbs.
Bake the meatballs: Bake at 350°F for 20–25 minutes, until browned outside, firm to the touch, and 165°F internally. Set aside, reserving a small amount of the pan drippings.💡 Tip:If you don't have a meat thermometer, these will finish cooking in the sauce. Bake until firm on the outside and no pink shows on the inside.
Roast the spaghetti squash: While the meatballs bake, cut the squash in half lengthwise and scoop out the seeds. Brush the cut sides with a little of the reserved meatball fat — this adds richness and keeps the squash from tasting watery. Place cut-side down on a baking sheet and roast at 350°F for 35–45 minutes, until a fork pulls the flesh into spaghetti-like strands. Undercooked squash won't shred properly and turns mushy instead of stringy.
Heat the sauce: Warm the marinara in a skillet over medium-low heat. Add the meatballs and simmer 5–10 minutes so they absorb flavor and stay juicy.
Assemble: Shred the squash into strands with a fork. Top with marinara and meatballs, add the provolone, and let it melt slightly before serving.