Place the potatoes in a large pot of cold water, cook until potatoes are tender, but still firm, 12 to 15 minutes. Drain, let cool then dice potatoes. Potatoes can be boiled up to 2 days in advance and refrigerated.
Par boil diced zucchini & yellow squash cubes (they should still be crunchy). Par boil whole sugar snap peas. Put par boiled vegetables into very cold water to stop cooking process. Cut sugar snap peas into small pieces about the same size as squash. Chop sweet onion and red pepper.
Add vegetables to diced potatoes. Pour dressing over potatoes and vegetables and mix gently. (I find the dressing recipe makes more than needed, so add about half and then add the rest slowly according to taste.)
For the dressing, combine the zest, juice, garlic, mustard, rosemary and salt and pepper in a blender (I use an immersion blendeand blend until smooth. With the motor running, slowly add the extra-virgin olive oil and blend until emulsified. Dressing can be made 1 day in advance and refrigerated. Bring to room temperature before using.