I own a multicooker that allows me to both brown and slow cook in the same appliance. If you do not have this functionality, start by heating a large heavy pot or French oven over medium-high heat. Sprinkle beef pieces fairly generously with salt. 

Add oil to the pan. When oil is hot and shimmery, add the beef pieces and brown so that each side has a beautiful browned crust to it. Remove beef to slow cooker and turn heat to the low setting. If you own a multicooker, turn heat to the brown/sauté setting and let it heat up for a minute or two. Add oil. When oil is hot and shimmery, add the beef pieces and brown so that each side has a beautiful browned crust to it. 

To the slow cooker, add the coconut milk, curry paste, lime zest and juice, fish sauce, brown sugar, cumin, and red pepper flakes. Stir to combine. Then add onion, bell peppers, mushrooms, garlic, ginger, and black pepper. Stir to coat all of the veggies with the coconut milk mixture.

Cover slow cooker and turn heat setting to high. Cook for 4 to 5 hours, or until beef is fork tender and falling apart. Remove beef to a cutting board and pull apart using two forks. Add beef back to the slow cooker. Serve over hot rice, with scallions, cilantro, and lime wedges on the side.