French Cod en Cocotte with Tomatoes

INGREDIENTS

2 medium Yukon Gold potatoes (12 ounces/350 g total)
6 plum tomatoes, peeled, seeded, and chopped
1 1/2 ounces (45 g) Spanish chorizo, cut into 1/8-inch (3-mm) dice
12 small black olives, such as Niçoise, pitted
1 large garlic clove, chopped
Four 4- to 5-ounce (125- to 150-g) skinned cod fillets
4 thyme sprigs
1/4 cup (8 g) chopped parsley
4 tablespoons (60 g) unsalted butter, sliced
3 tablespoons extra-virgin olive oil
2 tablespoons fresh lemon juice
Sea salt and freshly ground pepper