Meanwhile, heat oil in a large pot over medium-high heat. Add mushrooms, leek and garlic; cook, stirring occasionally, until the leek is soft and starting to brown, about 4 minutes. Whisk milk and cornstarch in a small bowl and add to the pot. Cook, stirring constantly, until the mixture thickens, 3 1/2 minutes. Stir in spinach, jalapeños, goat cheese, salt and pepper; cook until the cheese melts and the spinach wilts, about 2 minutes more. Remove from heat. Stir in the chicken.