Melt the butter in a small saucepan, add the sliced almonds, and stir continuously until they are browned and smell toasty. Spoon the nuts into a small bowl to use later.
Put the fresh spinach in a large mixing bowl, and add the sliced red onion.
Wash, core, and thinly slice the apple. Heat a tablespoon of olive oil in a large skillet, then add the slices of apple and fry for 2 to 3 minutes.
Whisk the brown sugar and two kinds of vinegar together in a small bowl, then pour into the skillet with the fried apples. Use a spatula to get all the sugar out of the bowl. Gently turn the apples over, trying to coat each apple with the vinaigrette. Turn the heat down to medium and cook for 1 or 2 minutes.
Pour the warm vinaigrette and apples over the fresh spinach and toss to combine.
Add red onion, toasted almonds, dried cranberries and crumbled feta, and toss gently. Serve immediately.