If using oven: preheat to 475˚F/250˚C, and put oven rack as high as it will go. If using grill, heat the grill.
Cut the stem and blossom ends off the squash. Now cut the squash into quarters lengthwise, then into pieces about 2 inches long.
In a medium bowl, combine the lemon juice, olive oil, mint, and garlic salt, and mix well. Toss the squash in this mixture, making sure all surfaces are well coated.
If using oven: arrange the squash on a flat pan (I use a quarter sheet pan) in a single layer. Leave any liquid behind in the bowl. Roast the squash, turning it after 15 minutes. Roast about 30 minutes total, until it is slightly browned and tender.
If using grill: place the squash pieces on the grill. Again, reserve any liquid in the bowl. Grill the squash uncovered, turning occasionally, until charred in spots and crisp-tender – 8-9 minutes total.
Return the cooked squash to the bowl with any leftover liquid. Mix it gently, and then sprinkle in the Feta cheese, and mix again.