Combine lager and honey in a small saucepan over medium heat.
Bring to a simmer and reduce until syrupy and caramelized, about 10 minutes, stirring occasionally. If you're reducing for a sauce, you're looking for nappe. It coats the back of a spoon, and when you draw your finger through it, it leaves a distinct line.
Remove from heat and let cool slightly. It will thicken as it cools.
Slice brie into cracker-sized portions.
Roughly crush the salted peanuts.
Place a slice of brie on each cracker.
Drizzle warm beer honey over the brie.
Top with crushed salted peanuts.
Arrange on a board and serve immediately.
Notes
Storage: assemble just before serving. The beer honey keeps in a sealed jar in the fridge for up to 2 weeks; warm gently before drizzling. Wrap leftover brie tightly and use within 3 days.