Preheat oven to 200C (390F), fan bake.
To cook lentils, add them to a saucepan with plenty of water and one bay leaf (don't worry if you don't have any, but if you do that's awesome).
Bring to a low boil and cook for approximately 20 minutes, or until cooked but still with some resistance and holding their shape (you'll have to taste a few to check). I start checking from 15 minutes, and if they're still too firm or chalky, give them another 4-5 minutes at a time before checking again.
Once cooked, drain in a sieve and run briefly under cold water. Leave for 5-10 minutes to drain well and continue cooling.
Discard the bay leaf.
Combine lentils, 1 tsp olive oil, red onion, mint leaves and cranberries in a mixing bowl. Add a good grind of salt and pepper to season.