Melt 3 tablespoons of unsalted butter in a fireproof pot with a tight-fitting lid.

Sauté the thinly sliced onions in the melted butter over medium heat, stirring frequently, until they turn golden brown.

Split the romaine lettuce hearts lengthwise and rinse under running water.

Add the wet lettuce halves to the pot on top of the sautéed onions.

Add the prepared carrots, turnips, and potatoes to the pot.

Cut the remaining 1 cup of unsalted butter into chunks and distribute evenly over the vegetables.

Cover the pot tightly with the lid and cook over low heat for approximately 1 hour, or until a knife easily pierces the potatoes.

Shake the pot occasionally to prevent scorching.

Ensure sufficient liquid from the lettuce; if necessary, add a small amount of hot water periodically to maintain moisture.

Add the asparagus tips, shelled peas (or sugar snap peas), and string beans to the pot.

Season with salt and freshly ground black pepper to taste.

Cover the pot again and cook for an additional 15 minutes, or until the green beans are tender.

Serve the Macedoine of Vegetables immediately.