Something of a secret culinary spot in Leblon, Formidable is a charming bistro with only a handful of tables inside (and a few tables out front). It has a chalkboard menu featuring beautifully executed French bistro classics such as boeuf bourguignon (beef stew braised in red wine), magret de canard (seared duck breast) and thon béarnaise (grilled tuna with béarnaise sauce).
Aside from the excellent main courses, don't skimp on dessert (profiteroles, crème brûlée). There are good wine selections as well. Come early to avoid a long wait.