While your water comes to a boil, chop the potatoes into bite-sized chunks (about 1-inch pieces so they cook evenly), trim the asparagus and cut into 2-inch lengths, trim the green beans and cut into 2-inch pieces, mince the garlic cloves, dice the red onion, and roughly chop the parsley. Toast the walnuts in a dry skillet over medium heat for 3-4 minutes, stirring occasionally, until fragrant and lightly browned—this deepens their flavor and creates better crunch. Set the toasted walnuts aside to cool.

Bring a large pot of salted water to a boil and add the chopped potatoes. Cook for 8 minutes, then add the green beans and asparagus pieces to the same pot. Continue cooking for 4-5 minutes until all vegetables are just tender but still have a slight bite—don't overcook or they'll become mushy. The potatoes should be fork-tender and the green vegetables bright green.

When the vegetables are done, drain the potatoes into a colander and set aside. Transfer the green beans and asparagus to a bowl of ice water to stop them from cooking further and to set their bright green color. While the vegetables cool, whisk together the olive oil, red wine vinegar, Dijon mustard, minced garlic, and black pepper in a small bowl until the dressing is emulsified and well combined.

Drain the cooked potatoes and place them in a large serving bowl. Pour half of the vinaigrette over the warm potatoes and stir gently so they absorb the dressing. Drain and pat dry the green beans and asparagus from the ice water. Add the cooled green vegetables, diced red onion, chopped parsley, and toasted walnuts to the potatoes. I like to add the remaining dressing gradually and toss gently to avoid breaking up the vegetables—you can always drizzle a bit more if needed for your preferred level of coating. Season with the salt and toss once more. Serve immediately or at room temperature.