Summer Squash Toasts With Smashed Feta

Name: Summer Squash Toasts With Smashed Feta
Ingredients: 2 summer squash (about 1 pound in total); 2 tablespoons plus 2 teaspoons lemon zest, divided (plus more to taste); 2 tablespoons plus 2 teaspoons extra-virgin olive oil, divided; 1/4 cup shelled, roasted pistachios, roughly chopped or smashed with a knife; 1 pinch fine sea salt, plus more to taste; 1 pinch freshly ground black pepper, plus more to taste; 5 ounces feta (a block packed in brine preferred); 1/2 cup full-fat sour cream (or Greek yogurt); 4 large slices from a loaf of crusty bread (such as ciabatta or sourdough), about 1/2-inch thick; 1 cup baby arugula (or watercress or a mix of leafy herbs)