To make the dough, place the flour, salt, and baking powder in the bowl of your food processor and pulse a couple of times.
Add the oil and warm water and process until the dough starts to come together. Tip it out onto a piece of parchment paper, form into a disk, wrap well, and set aside to rest for two hours.
To make the filling, dice the onion as finely as possible (alternatively, cut it into quarters and throw into the food processor/blender and chop until almost, but not quite pureed).
Heat a wide skillet over medium-high heat. Wait for the pan to get hot. Add the meat and brown well, breaking it down into small pieces with your spatula/spoon as you go. After a couple of minutes, add in the onion, mix around, and continue cooking over high heat for another few minutes, then reduce the heat to medium-low and cook until you see no more red in the meat and the onion doesn’t taste raw. Season with salt and pepper to taste.
Let the meat mixture cool for 10-15 minutes, then drain well and mix in a bowl with the eggs and breadcrumbs.
Once the dough has rested, divide it into two pieces. Roll the first piece out, keep the second piece wrapped in the meantime. I like to roll the dough directly on a large piece of parchment paper to prevent it from sticking, but you can do it directly on the counter, or on a board, etc. Roll it approximately ⅛ inch thick (3mm). This dough is pretty forgiving, but too thin and it will tear, but too thick won’t get you great results either.
Cut the dough into circles and place a spoonful of filling in the centre of each, then use a dumpling maker to seal.
Heat a pot of water with a couple of tablespoons of salt. When the water is boiling rapidly, drop in a group of kreplach. Don’t overcrowd the pot, they need some space, so do it in batches. The kreplach will rise to the top after a minute or two. At that point reduce the heat and cook for another 2-3 minutes. Remove and repeat until all the kreplach have been cooked.