Eggs en Cocotte with Tomatoes

• 1 onion (optional), plus butter for cooking
• 2 ripe tomatoes
• 1 tablespoon plus

If using, chop the onion finely and soften it in butter without browning it. Set aside. Remove the base from the tomatoes and dip them very briefly in boiling water. Peel them immediately, cut them in two, and remove the seeds.

Dice the tomatoes and sauté them in the same pan with half the butter (2 teaspoons