Boil the vegetables and eggs. Place unpeeled potatoes and carrots in a pot of cold salted water. Boil 25–30 minutes until a knife slides in easily. Hard-boil eggs separately for 10–12 minutes. Transfer all to ice water to cool. Peel and refrigerate until ready. This can be done the day before. Dice everything uniformly. Cut potatoes, carrots, eggs, chicken or sausage, and pickles into small even dice about ½ cm. Chop the onion as finely as possible. Uniform cutting ensures every tartlet has a little of everything.
Make the dressing. Whisk together mayonnaise, Dijon mustard, and pickle brine in a small bowl until smooth.
Combine the filling. Add all diced ingredients and peas to a large bowl. Pour the dressing over and fold gently until everything is evenly coated. Season well with salt and black pepper. Taste and adjust.
Chill the filling. Cover with plastic wrap and refrigerate at least 1 hour or overnight. This firms up the filling and deepens the flavour. Do not skip this step.
Fill the tartlet shells. Remove shells from packaging just before serving. Using a small spoon or piping bag, fill each shell generously, mounding slightly above the rim.
Garnish and serve. Top each tartlet with a small dill sprig, a pickle slice, or a tiny piece of egg. Arrange on a platter and serve. Fill no more than 30–45 minutes before serving to keep shells crispy.