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.