Recipe for Disaster: Freeing Evil Dave

Optional: West of Edgeville's bank, talk to Doris.

Head into the house's basement.

Talk to Evil Dave.

Speak to Doris, upstairs in the house. If you miss this step, the hellrats will only drop ashes.

Head back in the basement, interact with your cat and select "Chase-Vermin". (Switch off 'Hide Familiar Options' in Gameplay->Combat & Action Bar-> 'Choose Option' Menu)

As your cat catches the rats, pick up the spices that drop.

Do so until at least six doses of each of the four spices are collected.

Make Dave's Evil stew by combining doses of each spice on the stews.

For each of the spice collected, test each of them individually, a max of 9 stews is needed if done correctly.

Start with 1 dose of any spice on the stew, then use the spicy stew on Evil Dave (right click and use).

If he says one spice is right, note the number of dose using Spice type below.

(Skip if 1 dose is correct) Use 2 dose of spice on the stew, then use on Evil Dave, repeat step 2. Do not try for three spices, if 2 doses is wrong then it is 3 doses.

Repeat for each spice.

Once all 4 types of spice dosage are confirmed, use it on another stew with corresponding doses, then use on Evil Dave one last time.

This method is sort of like a game of Mastermind.

After determining the right combination, have Evil Dave taste it and finish the dialogue. This won't consume the stew.

Head to Lumbridge and give Evil Dave the stew.

Subquest complete!