Place onion slices in a small bowl and add 3-4 tablespoons of water (enough to roughly cover the onion) and rice vinegar. Set aside for about 10-15 minutes while preparing other ingredients. Give it a quick rinse with water before using.
Slice squid into either bite-sized pieces, strips or thin rings.
Bring a large pot with plenty of water to a boil. Then add the shrimps and poach for about 1-1½ minutes (depending on their size) if they are still in their shells or under a minute if they are already peeled. Transfer to a bowl with cold water to stop the cooking process. Peel off the shells once they are cool enough to handle.
Let the water come back to a boil, and then add the squid slices. Poach for about 45-60 seconds (depending on the thickness) or until just cooked through. Transfer to a bowl with cold water to stop the cooking process.
Add fish sauce and sugar to a small bowl. Stir well to combine or place the bowl in the microwave and microwave for about 30 seconds to fully dissolve the sugar. The mixture will be slightly thick. Add the lime juice and bird's-eye chili (if using).
To a large mixing bowl, add julienned green mango, carrot, onion, shrimp, squid, cilantro and mint (or Thai basil). Drizzle the dressing and shallot oil, then give everything a toss.
Transfer to serving plates and sprinkle roasted peanuts and fried shallots generously on top. Serve right away.