Preheat oven to 375°. Position oven rack in bottom third of the oven. Spray a 2-quart straight-sided enameled cast-iron Dutch oven with cooking spray.
In a 10-inch cast-iron skillet, melt butter over medium heat until bubbly. Whisk in flour; cook, whisking constantly, for 2 minutes. Slowly whisk in milk until smooth. Cook, whisking frequently, until sauce thickens, 10 to 12 minutes. Remove from heat.
Slowly whisk in Cheddar and Monterey Jack until melted and smooth. Whisk in cream cheese until melted and smooth. Whisk in pimientos, mustard, hot sauce, and salt until well combined. Gradually whisk in egg yolks until well combined. Transfer mixture to a large bowl.
In a medium bowl, beat egg whites and cream of tartar with a mixer at high speed until medium peaks form. Gently fold one-third of egg whites into cheese sauce. Repeat process twice with remaining egg whites. Gently spoon mixture into prepared Dutch oven, spreading evenly on top and leaving 1 inch of headspace at top of pot. Wipe inside rim of pot clean.
Bake until puffed and golden brown, about 30 minutes. Serve immediately.
Notes
KITCHEN TIP: The soufflé will rise dramatically as it bakes, but it will begin to deflate as soon as it comes out of the oven; this is normal.
Recipe by Southern Cast Iron at https://southerncastiron.com/pimiento-cheese-souffle/