Prep all the veggies: dice carrot, onion, shallot and thinly slice scallion.
Cut shrimp into small pieces.
Place a pan over medium high heat. When the pan is hot, add oil and add onion. Stir and cook for 2 minutes until fragrant, then add carrot. Continue to stir and cook for another 2 minutes. Transfer to a plate lined with paper towel to cool.
Add some more oil to the pan if needed and then add shrimp. Stir and cook over medium high heat for about 1 minute until shrimp is about 80-90% cooked. Set aside on a plate lined with paper towel to cool.
Add oil if needed and then add crab meat. Stir and cook over medium high heat for about 1 minute. Set aside on a plate lined with paper towel to cool.
When onion, carrot, shrimp and crab have cooled down, add them to a mixing bowl. Add mayonnaise, shallot, scallion, fish sauce and a generous amount of black pepper to the bowl and mix well.
Place one spring roll wrapper on a clean cutting board or working surface. Put 1 1/2 to 2 tablespoons of filling on it, shape into a log. Fold the top corner of the wrapper near you over the filling, fold left and right corners over the filling, and then roll it up away from you. Place on a tray or plate, seam side down. Repeat with the remaining filling and wrappers.
Heat at least an inch of oil in a frying pan. When the oil reaches 350F, add the spring rolls and fry over medium-medium high heat until golden brown and crispy (just 3 minutes or so). Only fry 4-6 spring rolls at once. Transfer fried spring rolls to a colander lined with paper towel, place them in an upright position to drain excess oil.
Whisk together all ingredients for the spicy mayo dip.
Serve spring rolls with spicy mayo dip, lettuce and mint.