Make the vinaigrette

Shake ½ cup apple cider vinegar, ⅓ cup olive oil, 3 tablespoons maple syrup, 1 teaspoon Dijon mustard, 1 grated or finely chopped garlic clove, 1 teaspoon kosher salt, and fresh black pepper to taste in a jar with a lid, or whisk in a small bowl until emulsified. The dressing will keep up to 5 days in the refrigerator.

Make the nuts

Put the almonds or cashews (or a mixture of both) in a medium skillet (8-10 inches). Place the pan over medium heat until the almonds begin to toast, 2-3 minutes. Stir in the brown sugar, water, salt, and cayenne (it will sizzle). Cook until the water is evaporated, stirring frequently. Transfer the nuts to a plate to cool and harden.

Make the salad

Chop the broccoli and cauliflower florets into small ½-1-inch pieces. You can also pulse them very briefly in a food processor, but don't overdo it. Transfer to a large bowl. Add the kale and green onions to the bowl and toss together.

Pour the dressing over the vegetables and toss until they're evenly coated. If you have time, let the salad sit 15-30 minutes to marinate. Taste the salad and add more salt, if needed. 
To serve, crumble the feta cheese over the salad and top with the nuts.