Heat a large nonstick skillet over medium heat. Add first 6 ingredients; cook 2 minutes or until fragrant, stirring frequently. Remove from heat; cool 5 minutes. Place spice mixture in a food processor; add lemongrass, ginger, garlic, and 1/4 cup broth. Process 1 minute, scraping sides occasionally; set aside.
Heat oil in a Dutch oven over medium heat. Add onion; cook 3 minutes or until tender, stirring occasionally. Stir in spice mixture; cook 1 minute, stirring constantly. Add beef, and cook 3 minutes, stirring frequently. Stir in remaining broth and water, scraping pan to loosen browned bits. Stir in tamarind paste, salt, and potatoes; bring to a simmer. Cover, reduce heat, and simmer 1 hour or until beef is very tender. Remove from heat; stir in coconut milk.