From tip.it
"1. Talk to Doris in front of the house west of Edgeville's bank. Ask if you can see Dave. She will let you go see him.
2. Open the trapdoor and go down. Ask Dave what food you gave him. He asks why. Tell that you have to get the food to get the magic right. He will tell you that he had a very evil stew.
3. Talk again to Doris, and she will tell you that all her spices were eaten. You have to make your cat hunt the Hell-Rats in the cellar and get spices. Note that after your cat eats 4-8 Hell-Rats, it turns into a Hellcat (of the same growth/physical stage it was at, including a possible Hell Kitten).
NOTE: If you want to skip the trial and error of finding the correct amounts of spices, click on "Get QuestHelp" at the bottom right of the Evil Dave quest journal and log onto the RuneScape quest help page. Then spend 3 QuestHelp points to view the spoiler. Make sure you've logged out into the lobby after reaching this part, or it will not give you the correct hint.
4. Mix them into the stew until you get the right dosage of every spice. To check if a spice amount is right, talk to Dave and he will tell you how many spices are right in the stew. You should try the spices one by one for example stew with only 1 yellow spice, stew with 2 yellow spices, stew with 3 yellow spices and write the number of that spice down when he accepts the stew. Then do this with each spice (there are 4 different spices: red, brown, yellow, and orange) until you find the correct number of each spice is right for the recipe. When you get the recipe right, Dave will not eat the stew like normal, but he will give it back to you. The recipe differs for each player.
5. After you have found the right combination, take it to Lumbridge and give the stew to Dave to free him.
NOTE:Clicking on "Get QuestHelp" at the bottom right of the Evil Dave quest journal will take you to the quest help website, and provide you with the correct combination, without the need for trial and error.
REWARDS
1 Quest Point
7000 Cooking Experience
Ability to make a Spicy Stew
Ability to own a hell-cat
Increased access to the Culinaromancer's Chest"