On this page of the Assassin's Creed Valhalla guide you will find a detailed walkthrough for Toast to Your Wealth side mission.
Rewards: 1200 XP.
In the place marked in the picture above, you will meet a man who sells potions for various ailments.
During the conversation you can try one of his potions. However, as it turns out, the alchemist does not have the necessary ingredients in his warehouse so you will have to get lichen and bear testicles by yourself.
You can collect the lichen right away. Just smash the wooden lid covering the well right next to the alchemist's hut. Jump downstairs to collect the required ingredient.
You will find the bear's lair near the alchemist's hut, in the place marked on the picture above.
Kill the animal to get the necessary ingredient. You can also collect lichen inside if you have not taken it from the well before. Go back to the alchemist and drink the potion he prepared.
Eivor will wake up in a completely different place, in a location called Besuncen Tor in the Gratnebridgescire area. Take the opportunity to collect the loot that is in this underground cavern, then go back to the alchemist and talk to him to finish this task.