Heat 1 teaspoon of the olive oil in a large skillet over medium heat. Working in batches if necessary, add the spinach and toss until just wilted. Transfer to a colander and squeeze out the excess water.

Wipe out the pan and heat the remaining 2 teaspoons olive oil over medium heat. Add the shallot and salt and cook, stirring occasionally, for 5 to 8 minutes, or until soft. Stir in the garlic and sautéed spinach, then add the cream, mustard, and cornstarch mixture and stir until combined. Stir in the lemon juice, season to taste with more salt and pepper, and serve.