Shave the squash lengthwise into long, wide ribbons using a vegetable peeler. Alternatively, slice the squash as thinly as possible. Thinly slice the onion. Combine squash, onions, and roughly chopped or torn basil in a bowl. Set aside.
To make the dressing, whisk lemon juice and yogurt in a small bowl. Drizzle in olive oil while whisking constantly, until emulsified. Season with salt and pepper to taste.
Toss squash with the dressing, toasted almonds and crumbled feta. Serve immediately.