In the Instant Pot®, combine the shells, water, 1½ cups (350 ml) of the cream, 4 tablespoons (60 g) of the butter, and the mustard. Season with 1 teaspoon salt and ½ teaspoon pepper. Give the mixture a stir, then lock the lid in place and turn the valve to Sealing. Press the Pressure Cook button and set the cook time for 7 minutes at high pressure.
While the pasta cooks, make the bread crumbs, if using. In a frying pan over medium heat, heat the oil. Add the bread crumbs and parsley, season with salt and pepper, and cook, stirring constantly, until the crumbs are well toasted, about 3 minutes. Remove from the heat.
Let the steam release naturally for 10 minutes, then turn the valve to Venting to quick-release any residual steam. Carefully remove the lid, add the cheeses, the remaining ½ cup (120 ml) cream, and the remaining 2 tablespoons (30 g) butter, and stir to mix well. Taste and adjust the seasoning, if needed.
Divide among individual bowls, top with the herbed bread crumbs, if using, and freshly ground black pepper, and serve.