To make pickled red onion: place the sliced red onion in a small, but wide bottom bowl. Pour red wine vinegar (or white wine vinegar) into the bowl until most of the onion is submerged. Set aside and occasionally stir to make sure all the onions get pickled.
Wash all your vegetables under running water. Bring a large pot of water to a boil. While the water is heating up prepare an ice bath. Fill a medium large bowl with ice and cold water, set aside. This will serve as a method to shock the vegetables and prevent them from continuing to cook.
Once the water is boiling drop the asparagus spears into the water. Boil until tender (3-4 mins depending upon thickness). You can spear an asparagus with a fork to see if it meets your standard of al dente. After two minutes of cooking the asparagus add the peas (they need about 1-2 minutes in the boiling water).
When the asparagus is to your liking, drain the vegetables into a colander and immediately place in ice water bath for 2 minutes. After 2 minutes, drain the vegetables in a colander. Slice the asparagus spears to about ¾-1 inch pieces.
While the water is boiling you can start slicing the other vegetables and putting them aside. Slice the radishes and yellow squash as thin as possible, use a knife or mandolin. (Or if you have a food processor with the slice attachment, feel free to use that.)
In a small bowl, put ½ Cup water and 1TBSP sugar and mix well. Add the sliced radishes to get rid of some of the bitter taste. *If you do not mind the peppery taste of radishes then feel free to skip this step.
To make the dressing: Squeeze fresh lemons (depending upon size, will need 1-2 lemons) into a small bowl and strain the seeds. *Note you can use bottled lemon juice but fresh lemons are more flavorful
Place all the dressing ingredients into a blender. Blend until well combined.
To Assemble: Place the arugula in a bowl or on a large platter. Place all the prepared vegetables (sliced and blanched) on top. Crumble the goat cheese and sprinkle with the sunflower seeds. Serve with the dressing.