1. Using kitchen shears, cut the chiles into very thin strips. In a stockpot, heat the olive oil until shimmering. Add the onion, garlic and chiles and cook over high heat until the garlic is just beginning to brown, about 7 minutes. Add the stock and bring to a boil. Add the collard greens by large handfuls and stir to wilt each batch before adding more. Season with salt and pepper and bring to a simmer. Cover and cook over moderately low heat until the collards are tender, about 40 minutes.
2. Transfer 1/2 cup of the cooking liquid to a small bowl and stir in the tamarind concentrate, then stir this mixture into the collards. Season with salt and pepper. Transfer the collards and their liquid to a large bowl and serve.
Make Ahead: The greens can be refrigerated overnight.