It's not easy to find small potatoes that are uniform in size (and therefore cook at the same rate), so lift them out of the water in batches as they become tender. The recipe is from Dana Ewart and Cameron Smith, chefs and caterers in Canada's Okanagan Valley, in British Columbia. They serve it as part of their open-air feasts at God's Mountain Estate, a vineyard and B&B near Penticton.
Bring potatoes to a boil in a medium pot of well-salted water just to cover. Reduce heat and simmer until just soft when pierced (or cut one in half and try it).
Whisk 1/2 cup oil, the vinegar, 1/2 tsp. salt, and 1/4 tsp. pepper in a small bowl.
Sauté onions in remaining 2 tbsp. oil until translucent. Add garlic and cook 30 seconds, then add corn and heat until just warmed through. Season with remaining 1/2 tsp. salt and 1/4 tsp. pepper and let cool.
Drain potatoes and let stand until just cool enough to handle. Taste a shred of skin; peel if bitter. Cut potatoes into bite-size pieces and toss with 1/2 of vinaigrette.
Add cooled corn mixture, tomatoes, and marjoram. Drizzle with remaining vinaigrette to taste and gently mix together.