Bring ½ a cup vinegar, ¼ cup sugar and ½ tsp. salt to a boil in a saucepan; then, lower the heat and simmer, stirring occasionally, for about 5 minutes. Add 1 sliced onion and simmer, stirring occasionally, for 2 minutes. Transfer to a bowl or mason jar and cool completely. Refrigerate and use within a week.
Lentils are not complicated. But they are delicate. Treat them with care and you’ll get tender lentils every time. For a cup of lentils, you’ll want three cups of water. Bring the water to a boil in a small saucepan; add salt like you would to pasta water. Then add the lentils, cover the pot and reduce to a simmer. You want the lentils to be just tender, but not overcooked. They will take about 20-25 minutes. Taste them after 20 minutes. They should be tender enough to melt in your mouth. If they are still crunchy, cook them for another five minutes. Drain them and toss with a tablespoon of olive oil. Lentils like this will keep in your fridge for up to a week.
Meanwhile bring a large pot of salted water to a boil. Add the potatoes and cook for 10 to 15 minutes, until you can easily pierce them with a fork.
Remove the potatoes from the water and let them cool slightly. Using the bottom of a bowl or cup, or the palm of your hand, smash the potatoes until they’re just crushed to expose the inside, but not so much that they fall apart.
Heat ¼ cup oil in a large skillet over medium-high heat. Place the potatoes in the skillet in a single later (you may need to do it in batches). Sprinkle with salt and. Fry each side until brown and crispy, about 5 minutes.
Whisk together the sherry vinegar and mustard and 3 Tbsp oil. Toss 2 cups of cooked lentils with vinaigrette, smashed potatoes and ¼ of pickled red onions. Season with salt and pepper to taste.