Special equipment: a straight-sided 3-quart tall glass bowl or four 16-ounce wide-mouth glass jars with lids
Toss the onions with 2 tablespoons each vinegar and cold water and a large pinch of salt in a small bowl. Let sit for at least 30 minutes, tossing occasionally. (The onions will turn bright pink.) Strain.
Meanwhile, put the eggs in a medium saucepan, and add water to cover by an inch. Bring to a boil, add the peas, cover and remove from heat. Let sit for 10 minutes. Run the eggs and peas under cold water to cool. Remove the shells, and cut the eggs into wedges.
Pulse the milk, cream cheese, remaining 1 tablespoon vinegar, garlic and 1/2 teaspoon salt in a blender until smooth. Pour the mixture into a small bowl, and stir in the sesame seeds, poppy seeds and onion flakes. Refrigerate the dressing, covered, until ready to serve.
Using a 3-quart tall glass bowl for 1 large salad or four 16-ounce wide-mouth glass jars with lids for 4 individual salads, make a first layer with half the arugula, then add the following in separate layers: the radishes, half the broccoli, the smoked salmon, the remaining arugula, the remaining broccoli, the pickled onions, bagel chips, snap peas and eggs. Sprinkle the top with poppy seeds. (The salad can be put together, covered and refrigerated for up to 2 hours before serving.)
If making 1 salad, just before serving, transfer it to a large bowl, toss with about half the dressing and season with salt and pepper. If making 4 salads, add dressing to each jar, season with salt and pepper, cover with the lid and shake to coat. Serve the remaining dressing on the side.
To achieve pretty, defined layers, be sure to layer more on the sides. It's OK if there isn't enough of an ingredient to fill the middle.