Cut the onion in half and then into slices, in a bowl whisk vinegar and sugar together until the sugar has dissolved, add the red onion slices, and leave in the liquid for 30 minutes.
Wash the potatoes leaving skin on, cut in 4 quarters and boil in salted water for 5 to 7 minutes or until cooked but still firm.
Cut the kernels off from the corn cobs by slicing down each side of the corn on the cob so all the kernels fall off.
In a small saucepan, heat up the water and lemon juice. Now beat in one butter cube at a time, and whisk after each one, continue until you have used all the butter cubes. The sauce should be smooth and pale yellow.
Now mix the warm potatoes with lemon butter dressing in a big bowl, take the onions out of the vinegar, and toss in along with the corn kernels and baby spinach, season to taste with salt and pepper. Serve right away.
- To make a perfect beurre blanc, whisk cold butter cubes into simmering lemon juice and never let it boil! Overheating the sauce can cause it to split.
- Don’t delay in tossing the warm potatoes in the butter- lemon dressing to prevent the butter from solidifying.