Line a baking sheet with parchment paper and set it aside within reach of your stove. Add the pecan halves, granulated sugar, butter, cinnamon, and a pinch of salt to a small saucepan.
Place the saucepan over medium heat and stir constantly with a wooden spoon or silicone spatula. Cook for 3 to 5 minutes, stirring the entire time, until the sugar melts and coats the pecans in a glossy, amber colored glaze.
Immediately pour the candied pecans onto the prepared baking sheet and spread them into a single layer using two forks to separate any clumps. Let cool completely at room temperature for about 10 minutes.
Add the olive oil, apple cider vinegar, granulated sugar, poppy seeds, Dijon mustard, and a pinch of salt to a mason jar. Secure the lid tightly and shake vigorously for about 30 seconds, until the mixture looks fully combined and slightly thickened.
Wash the baby spinach thoroughly, then dry it completely using a salad spinner or by patting it dry with clean kitchen towels. Thinly slice the red onion into half moons, then separate the slices with your fingers.
Add the dried spinach to a large salad bowl. Scatter the dried cranberries, crumbled feta cheese, and sliced red onion evenly over the spinach. Once the candied pecans have fully cooled and hardened, break any large clusters apart with your hands and add them to the bowl.
Give the poppy seed dressing one more quick shake to recombine any ingredients that settled. Drizzle about two thirds of the dressing over the salad, reserving the rest to pass at the table. Toss the salad gently using salad tongs or two large spoons, making sure the spinach is lightly coated without becoming soggy. Serve immediately.