Toast the pecans: melt butter in a small skillet over medium heat. Add pecans and stir for 3 to 4 minutes until fragrant and golden. Season with a pinch of salt and cool completely on a plate before using.
Make the dressing: add apple cider vinegar, maple syrup, Dijon, salt, and pepper to a small jar. Shake to combine. Add olive oil and shake for 20 seconds until emulsified. Taste and adjust seasoning.
Prep the onion: soak thinly sliced red onion in cold water for 5 minutes. Drain and pat dry before adding to the salad.
Slice the apple just before assembling. Toss immediately in a small squeeze of lemon juice to prevent browning.
Assemble: add spinach to a large wide bowl. Scatter apple slices, dried cranberries, red onion, and cooled pecans evenly over the top. Crumble feta over last.
Dress and serve: drizzle two-thirds of the dressing over the salad and toss gently using wide tongs, lifting from the bottom. Add more dressing to taste and serve immediately.
