After evolving Kubfu into Urshifu and finishing the post-game story (where you catch Zacian or Zamazenta), there's more you can do to help Urshifu. Head inside the Master Dojo and you'll bump into Hop and Mustard having a conversation. Mustard reveals that Urshifu can Gigantamax, but you need an extra ingredient for the Max Soup.
Trouble is, Mustard doesn't tell you what this ingredient is or where to find it. Hop seems to have a clue though. Exit the dojo and head for the Forest of Focus, via the Soothing Wetlands. Hop will be standing by the entrance of the forest. Approach him to learn what's up. Oh, nothing much. Head inside the forest.
Forest of Focus
Go forward until you're almost at the first bridge. There will be a Petilil in the middle of the path. Go near it and Hop will eventually catch up. Seems like this Petilil has lost its parents. When you're back in control, follow the path on the right--the same path you should've taken to reach the Training Lowlands.
Keep going all the way until you've gone across the second bridge. Up ahead, you should see a Lilligant by the side of the road. Bingo! Now you have to lead the Lilligant back to the Petilil. The Lilligant will follow behind you, but it's a bit slow, so try not to go too fast when going across the bridge or around corners.
Make your way back to Hop, by the entrance. Then speak to him when Lilligant is nearby. As thanks for reuniting the parent and child pair, you'll receive some nectar from Lilligant. However, it doesn't seem to be the right ingredient. Chase after Hop, along the same path that leads to the Training Lowlands.
When you've gone past the body of water, you should see an Applin at the beginning of the branch on the right. Hop will catch up with you. But the pair of you end up scaring the Applin away. Go up to the berry tree in the dead end just ahead. Applin will fall from the tree and you'll receive its nectar. Nope, this isn't the right ingredient either. Hop!!!
After that wild goose, Hop finally figures out where you should be looking: Honeycalm Island, off the shores of the Isle of Armor. Open your map and use the flying taxi to fly to the Tower of Waters (if you unlocked it as a fast-travel location) or the Training Lowlands.
If you're at the Tower of Waters, get on your bike and head into the water on your left (when your back is facing the tower). In the distance, you should see an island with a tree in the centre. That's where you need to go. As usual, watch out for those annoying Sharpedo.
Otherwise, if you're coming from the Training Lowlands, you need to turn around, then go all the way forward, while keeping to the right. Keep going until you reach the beach at the far end of the zone. Turn left slightly and you should see an island with a tree in the centre. Get on your bike and paddle all the way to that island.
When you arrive at this island, head for the tree in the very centre. You'll find Hop standing in front of the tree. Hmm, there's something suspicious about that tree. When you're ready, inspect the tree and give it a good ol' shake. Wow, those are some crazy strong arms your character has!
Max Raid Battle
This is a solo Max Raid Battle, similar to the ones during the post-game story. Vespiquen can use Bug, Fighting and Flying-type moves. As such, Fire, Flying and Electric-types are recommended. Rock-types can deal quadruple damage, if you're confident they can take a punch in the face or two.
Successfully swat the oversized bug and you'll obtain some Max Honey. Looks like you hit the jackpot! Without further delay, fly back to the Master Dojo and speak to Mustard. Now you can feed Urshifu some Max Soup by speaking to the fella in the kitchen. It will cost 3 Max Mushrooms and 1 Max Honey each time though.
Additionally, now that you've learned everything the dojo has to offer, you can now battle Mustard at his full strength. Ooh...