Heat 2 tablespoons oil in a large skillet over medium-high heat.
Add sliced mushrooms; cook, stirring occasionally, until browned, 7 to 10 minutes.
Add chopped shallot, minced garlic, ¼ teaspoon salt and ¼ teaspoon pepper; cook, stirring constantly, until softened, about 30 seconds.
Stir in 1½ cups broth and bring to a simmer.
Add softened cream cheese; cook, stirring constantly, until melted and smooth, about 2 minutes.
Stir in rinsed butter beans and return to a vigorous simmer over medium-high heat.
Cook, stirring occasionally, until the beans are heated through and the sauce is creamy and slightly thickened, 2 to 4 minutes.
Remove from the heat and stir in ½ cup Parmesan, 2 teaspoons mustard and 1 teaspoon lemon zest.
Sprinkle with 2 tablespoons parsley and the remaining ⅛ teaspoon salt.
Garnish with additional lemon zest, if desired.
Serve with the toasted baguette.
