Shallots have an incredibly unique flavor. It is an onion that grows in clusters similar to how a garlic would. It tastes just like onion and garlic combined but with a milder and sweeter flavor. Because of their mild but full flavor profile shallots are popular ingredients in recipes all over the world.
Red onions have a milder flavor compared to white or yellow onions making it very close to the shallot's flavor profile. It is a great substitute for garnishes not only because of flavor but because of its appearance as well.
Scallions have a mellowed our onion flavor. It is great for garnishes and additional flavor on uncooked dishes. To get the full flavor, use only the bulb part of the scallion when cooking. The green hollow part of the scallion can be used as a garnish.
Leeks are quite similar in flavor as scallions are with shallots. Make sure to use the green part of the plant to get the full flavor. It is best to use leeks as a garnish on dishes to experience its crisp flavor. If your recipe requires it to be cooked, it is best to add it last or towards the end of the cooking process for it to retain its flavor.