Preheat the oven to 425˚F. Cut the squash in half widthwise and scoop out the seeds. Slice each half into ½”-thick rings and spread out in a single layer on a baking sheet. Depending on your squash’s size, you should get about 12 rings. Roast until the squash is just starting to brown, 15 minutes. When the squash is finished, let sit until cool enough to handle.
When the squash is cool, preheat a medium pot with 2” of high-heat sunflower oil until it reaches about 350˚F and aim to maintain that temperature throughout the cooking process.
Whisk together the remaining ingredients in a medium bowl. The batter should be thin enough to easily coat the squash but thick enough that an opaque layer clings to the ring. You can tweak the consistency by adding a splash of beer to thin or a sprinkle of flour to thicken.
Place the squash in a large bowl and add the cornstarch. Toss to evenly coat the squash. Place a wire rack on a rimmed baking sheet and set aside. Once the oil is ready, dip a ring into the batter to coat, then shake off any excess. Transfer to the hot oil and repeat with as many squash as will fit without overcrowding. Cook about 2 minutes, flip, and cook for another 2 minutes or so, until the squash is golden but not dark. Transfer to the wire rack to allow any excess oil to drip off. Repeat with remaining squash.