Coarse or kosher salt and freshly ground black pepperto taste
Preheat the grill to medium. Toss the broccoli and onions with the olive oil, season with salt and pepper, and grill, turning as the bottom sides become browned, for about 10 minutes total, until the vegetables are browned and tender. Remove them from the grill and roughly chop them.
Heat a heavy saucepan over medium-high heat. Add the farro and cook, stirring frequently for 2 minutes, until it is lightly toasted. Add the butter and the garlic and sauté for two more minutes so that the butter melts and the grains are well coated, and you can smell the garlic. Add the broth, season with salt and pepper, and bring to a simmer. Lower the heat to medium and simmer, stirring occasionally, until the broth has been absorbed and the faro is cooked through, but still has a nice chewy consistency, about 20 to 25 minutes.
Stir in the chopped grilled vegetables and serve warm.