Heat a small saucepan over medium heat and add the butter. Once it’s melted and starts to sizzle, add the sage leaves and cook until slightly golden on each side, about a minute per side. Remove the leaves and place them on a paper towel to drain.
Toss the greens, apples and feta together in a large bowl. I don’t slice the apples until right before serving – this way they won’t brown and the salad only takes seconds to put together.
To make the dressing, add the vinegar, syrup, garlic, mustard, salt, pepper and pepper flakes to a bowl or a mason jar and whisk to mix. Slowly stream in the olive oil while mixing, or add it to the jar and shake the dressing for 15 seconds.
Season the salad with salt and pepper and dress it before serving.