Heat 1/4 cup oil in a large skillet over medium until shimmering. Add garlic and cook, stirring, until fragrant and sizzling, about 30 seconds. Add beans and cook, stirring a few times, just until warmed through, 2 to 3 minutes.
Transfer mixture to a food processor. Add coriander seeds, tahini, lemon juice, salt, and remaining 1/4 cup oil. Pulse just until mixture is thick and creamy but not totally smooth; let cool completely. Transfer to a bowl. Refrigerate, covered, up to 3 days. Bring to room temperature, drizzle with oil, and sprinkle generously with sumac before serving.