Put potatoes in a 4- to 5-quart pan. Barely cover with water, put lid on pan, and place over high heat. When boiling, reduce heat and simmer until potatoes are tender when pierced, about 20 minutes.
Meanwhile, peel onion and thinly slice lengthwise. Mix with lemon juice.
Slice cheese; arrange in a shallow 8- by 11-inch casserole or 4 individual casseroles, each about 5 inches wide.
When potatoes are cooked, drain off most of the water and keep warm.
Broil cheese about 6 inches below heat until melted, about 5 minutes.
To eat, put potatoes, onion, and pickles on dinner plates. Spoon hot cheese, a few bites at a time, over potatoes. Add salt and pepper to taste. If cheese in casserole gets hard, return to broiler to soften, 3 to 4 minutes.