So, after 3 days of being unable to advance I decided to look it out on a walkthrough.
I am at the second part of the game, I put the ghost in the scarecrow and the farmer starts chasing it. Problem is, I didn't give him the letter before that so now I can not make mustard.

How was I supposed to to know in wich order I had to do this stuff? Is there really no way to advance like this? I don't want to restart the game.

What happens when you try to combine the ingredients? I don't remember ever playing without giving the letter to the farmer, so I'm not sure what effect it has on making the mustard. Are you using the right ingredients? Did you grind the radish before adding it to the bowl to combine with vinegar?

If the game won't allows you to combine the ingredients you may have to restart. Before you go on, however, did you write down the bugs color combination from earlier in the game? If not, you may not be able to access a location later, so if you restart the game, make sure to write the combination (or keep a save game there to check later) if you didn't before.

Nothing happens, seems the event of giving him the letter triggers the ability to obtain mustard from the mix.

Its no use.

Guess I will have to restart it. Such a shame, the game could be so great had it been tested properly. I still remember the bugs color combination, but I think I will come back for this game later after the bad taste washes away. Started other graphic adventure while waiting. But thanks anyway for your fast response!

That's a shame, I love the Kyrandia games (well, at least the first two) but they certainly have their share of design problems. Can't blame you for deciding to start playing something else, I hope you have fun with that game.

I am also stuck, I even went to an earlier save to talk about the mustard recipe and it will not make mustard. I just need ground radish and vinegar right? I don't know why it won't work :(

You may not need that recipe. Use a radish on the stone mill west of the house, scoop it up with the bowl, combine it with the vinegar and Bob's your uncle. At least, it worked for me.

I figured out the answer from watching a youtube video. Mustard isn't a potion, so you don't combine it in the cauldron. You just combine it in your inventory. I got confused as well because I think this is the first time in the game that you have to combine items in your inventory. In the first area of the game the only combinations you make are in the cauldron, so it conditions you to think that is the only way you can use one item on another.