Preheat oven to 350 degrees Fahrenheit.

Peel the potatoes and cut them in half length-wise. Coat them in olive oil and place on a pan lined with parchment paper.

Bake for 30-40 minutes, until they are soft (if you cook them too long they may brown a bit on the edges, which is fine but will make them look less egg-like). Once they are done, allow them to cool for a while (they will be very hot!).

Combine all other ingredients (except paprika) and mix thoroughly, then set aside. 

Once the potatoes are cool enough to handle, scoop out the middles (from the flat side) to make little cups where the filling will go. Add the scooped-out potato to the mayo/mustard/onion mixture and combine well. 

Fill the potatoes with the mixture using a spoon or a piping bag. Sprinkle the paprika over the tops of the potatoes.

Enjoy immediately or chill in the fridge to save for later.