Sprinkle roast evenly with 1/2 tsp. salt and 1/2 tsp. black pepper. Brown roast on all sides in hot oil in a Dutch oven over high heat.
Stir together flour, next 4 ingredients, and remaining 1 tsp. salt in a large bowl. Whisk in 1 cup warm water until smooth.
Add 2 cups beef broth to Dutch oven. Stir in flour mixture. Bring to a boil over high heat; cover tightly, reduce heat to low, and simmer 2 hours to 2 hours and 30 minutes or until meat is tender, stirring every 20 minutes.
Divide Roasted Corn and Garlic Chipotle-Cilantro Mashers evenly among 6 serving plates. Arrange roast equally on top of Mashers, and top with gravy mixture. Dollop each serving with sour cream, and sprinkle with chopped cilantro, if desired. Garnish, if desired.