In a small bowl, combine 1/2 cup of apple cider, 1 tablespoon of sugar, and 1/2 cup of water. Mix well. Submerge the onions in the bowl and let them sit for at least 15 to 20 minutes to pickle while you prepare the rest of the salad.

To toast the slivered almonds, place them in a dry skillet over medium heat, stirring frequently until golden and fragrant, about 3 to 5 minutes. Set aside to cool.

Separate the hard-boiled eggs into yolks and whites. Finely chop the egg whites and set them aside.

To make the dressing, combine the egg yolks, mustard, 3 tablespoons of apple cider vinegar, and 3 tablespoons of sugar in a food processor. Blend until smooth.

Cook the bacon in a skillet until crispy. Remove the bacon and keep the drippings warm in the pan. Chop or crumble the bacon into small pieces and set aside.

In a large bowl, toss the baby spinach leaves and croutons with the dressing. Drizzle the warm bacon drippings over the salad and toss again to lightly coat.

Drain the pickled red onions and scatter them over the salad along with the chopped green onions. Sprinkle the chopped egg whites, crumbled bacon, and toasted slivered almonds evenly over the top. Serve immediately, while the salad is still slightly warm, for the best flavor and texture.