Start by boiling the spaghetti in a large pot of salted water until it's al dente and slightly firm to the bite. You will know it is ready when it is tender but still has a little resistance. Be careful not to overcook it, or it will get mushy. This should take around 8 minutes.
Once the spaghetti is cooked, rinse the pasta under cold water to stop the cooking process. This also helps remove extra starch, which can make it sticky. Ensure to drain it well so your salad does not turn watery.
Slice the cucumber and onion thinly for perfect crunch. Thin cuts will provide good texture in the salad. If they are too thick, they may overpower the dressing and other flavors.
In a mixing bowl, whisk together Kewpie mayonnaise, Dijon mustard, and a splash of milk. The mixture should be smooth and creamy, with a hint of tang from the mustard. If it is too thick, add a bit more milk until the desired consistency is reached.
Fold in the diced leg ham and sprinkle in a pinch of salt and pepper. The ham adds a nice savory touch, and seasoning will enhance the overall flavor. Adjust salt as needed.
Gently toss the spaghetti with the veggies and dressing until everything is well coated. The pasta should shimmer with the creamy dressing. Avoid stirring too vigorously to prevent breaking the pasta.
Let the salad chill in the fridge for a little while, allowing the flavors to meld. It is best served cool. Taste and adjust seasoning before serving if necessary.