For the dressing: 

1. To a blender, add the vinegar, garlic, oregano, Dijon, 2 tablespoons lemon juice and salt. Blend and slowly add the olive oil while the blender is still going so it can emulsify completely. You can also add all the ingredients to a jar with a lid and shake until it emulsifies and all the ingredients have blended. Set aside. 

Make the Pasta Salad: 

1. Bring a large pot of water to a boil. Add a good amount of salt, 1-2 tablespoons, and cook the pasta 1 minute over al dente, according to the package instructions. Drain and set aside to cool completely. Drizzle ½ cup of the dressing onto the pasta while it’s warm to keep it from sticking and to soak up the flavor of the dressing. 
2. Prepare a large bowl with iced water. Place a medium pot over high heat and bring to a boil. Add salt to the boiling water then the asparagus and par-boil until tender, about 1 minute and 45 seconds. Add the frozen peas to the pot with the asparagus and cook another 30 seconds. Remove the vegetables from the boiling water with a slotted spoon or mesh strainer and transfer them directly to the ice water bath. Once the asparagus and peas are cooled, transfer them to a paper towel to dry. 
3. To a large bowl, add the pasta, asparagus, peas, marinated red onion, pistachios, celery, olives and herbs. Toss to mix completely. Drizzle about 1/2 a cup of the prepared Italian dressing over the salad and toss. Continue adding more dressing, a little at a time, until the salad is dressed to your liking. Taste and add more salt if needed. Crumble feta cheese on top and sprinkle with a pinch of red chili flakes (if using) and gently fold into the pasta salad. Serve immediately or refrigerate for 30 minutes to allow the flavors to meld.
4. Squeeze remaining 2 tablespoons lemon juice over the pasta and give everything one last toss just before serving.