Preheat oven to 400°F. Halve the acorn squash lengthwise and scoop out seeds. Brush cut sides lightly with oil and season with salt and pepper. Place squash cut-side down on a baking sheet and roast for 30–35 minutes, until very tender when pierced with a knife.

While the squash roasts, heat a large skillet over medium-high heat. Add the ground sausage and cook, breaking it up, until well browned and cooked through. Remove sausage to a bowl and set aside.

In the same skillet, add the diced red onion. Cook over medium heat until softened and lightly golden, about 4–5 minutes.

Add smoked paprika and cumin to the onions and stir for 30 seconds to bloom the spices. Add the diced apple and cook for 2–3 minutes, just until the apple loses its raw edge but holds its shape.

Add the diced red bell pepper and cook until just tender, another 2–3 minutes. Return the sausage to the pan, then fold in the cooked black rice. Stir gently to combine and heat through. Remove from heat and season with black pepper and salt as needed.

When the squash is fully tender, remove from the oven and flip the halves cut-side up. Divide the filling evenly among the squash halves, mounding slightly. Sprinkle pistachios over the top.

Return stuffed squash to the oven and bake at 375–400°F for 15–20 minutes, until the filling is heated through and the edges are lightly sizzling. For additional browning, broil briefly at the end if desired.

Finish with a small squeeze of lemon juice or a splash of apple cider vinegar, if using. Serve warm.