Add shallot and vinegar to a small bowl. Slowly whisk in olive oil to emulsify. Season to taste with salt and pepper.
Add mache and butter lettuce to each plate. Dress with the vinaigrette right before serving. Garnish each serving with pomegranate seeds.
You can substitute dried cranberries if you cannot find pomegranate. Dress the salad right before serving. If you serve it too early, it will get soggy for your guests.