Heat the oil in a large nonstick skillet over medium-high heat. Add the squash, jalapenos, onions, vinegar, 3/4 teaspoon salt and pepper to taste and stir to combine. Cover and cook until the squash starts to brown, stirring twice during cooking, about 6 minutes.
Remove the lid and continue to cook, stirring occasionally, until the squash is nicely browned and tender, another 6 minutes. Add the sage and garlic and cook for 1 minute. Season with salt to taste. Stir in the chives, transfer to a bowl or platter and serve.