Peel potatoes if you choose, cut in half lengthwise, and quarters lengthwise.
Cut quarters into 1/4" thick slices.
Add the slices to a bowl of cold water and swirl to remove extra starch if you want to, but this step is optional.
Dry well on a clean kitchen towel if you rinsed yours. If not, move to frying:
Heat a 10-12 inch heavy skillet over medium-high heat 2 to 3 minutes (use your senses to know when the pan is hot) then add the lard and heat until melted.
Add the potatoes and cover with a lid. Reduce the heat to medium and cook for 10 minutes. Don't move the potatoes for the first three or so minutes of cooking. Then stir occasionally after that.
Remove the lid and add salt and pepper.
Cook, stirring occasionally until the potatoes are soft throughout; another 8 to 9 minutes.
Add the diced onion and stir to combine. Turn the heat down to low if needed but keep an eye on your onions so they don't burn. Cook another 5 minutes to 7 minutes until the onions are translucent and soft. Taste for seasoning adjustments and serve warm.