This makes about 12-14 blintzes, depending on how generously you fill them.
If you don’t have any mashed potatoes around, make some. You need about 3 to 4 cups of them. Season them with butter, milk, salt and lots of pepper. While the potatoes are cooking…
Dice 1–1/2 large onions or 3 medium onions. In a frying pan, heat 2 TBL oil, and saute the onions on a low heat. They will get just a bit browned towards the end of the cooking, but this should be done on a low heat and take about 30 minutes. Season with salt and pepper while the cook, and when done, mix into the mashed potatoes. Taste the mixture: they should be peppery and not lacking in salt. Season them now if you think they need it! You can do the next step (making the wrappers) while the onions saute.
In a blender, combine 1 cup milk, 3 eggs, 2 TBL corn or canola oil, 1/2 tsp salt and 3/4 cup flour. Run on high speed for a minute to blend thoroughly. Heat a 6-7 inch frying pan on medium to low heat. A pan with sloped sides like an omelette pan or a round griddle with no sides work the best. As it cooks you will definitely want to lower the temperature to low.
Grease the pan lightly: I use a bit of butter/margarine on a paper towel. Pour 3 TBL batter into the middle of the pan all at once (see note), and rotate the pan to spread the batter in a circle, continuing until you have a circle that’s about 6 inches across. Continue letting it cook until the top surface is dry. If you need to, run a knife around the “wrapper” to release it and invert the pan onto a clean dishtowel. The wrapper should drop off; it will just have a slight hint of color, and if it doesn’t don’t worry about it. Repeat this step until you run out of batter. (If you make these in advance, be sure to separate them with waxed paper, 3 per layer distributed so they aren’t stacked perfectly and put them away in the refrigerator until you’re ready to use them: they will keep for a day or two.)
Assemble the blintzes. With the darker side of the wrapper face-up, put about 1/4 cup of mashed potatoes near the edge closest to you. Roll the wrapper over about an inch or two, fold the sides in and continue rolling to make a cylinder. Set it with the last edge down so it doesn’t pop open. The uncooked side of the wrapper should be on the outside. They can be refrigerated for a day or two, or frozen (see note, below).
When you are ready to serve the blintzes, heat some oil, butter or margarine in a frying pan. An electric frying pan works really well because it distributes the heat and you can cook a lot of them at once. Add the blintzes; as they get brown on one side, turn them. Potato blintzes can be cooked on 3 or even 4 distinct sides, so they are completely browned. Serve hot with sour cream!
NOTE: To measure the batter, 3 TBL is a scant 1/4 cup measure. Measuring the batter this way will give you wrappers of pretty uniform size. Perhaps you have a ladle this size, but do try to measure out the batter.
NOTE: Blintzes can be frozen, uncooked. Freeze them on a parchment-lined cookie sheet, not touching one another, then transfer to bags. That way you can defrost just what you need. Defrost them in the refrigerator overnight and cook as described above. My mother always made at least 100 at a time. It was a like a factory when she started doing this.
Finally, there are lots of other kinds of filling: cheese blintzes (filled with pot cheese or farmer cheese) are delicious. I’ve also had ones filled with mushrooms and kasha. I’m not a fan of fruit blintzes.