The important thing is that your preparations be palatable for Lord Krishna and that the ingredients be within the groups of ingredients already listed. You have asked me about sassafras and yes, it may be offered. Mushrooms are generally not offered, but there is no prohibition, there is no harm in them.
I hope that this has answered your questions. Please keep me informed as to the progress of your temple. Hope you are all well.
Your ever well-wisher,
A.C. Bhaktivedanta Swami

Letter to Harer Nama Los Angeles Dec 1st 1968

Because mushrooms grow in a filthy place, they are not usually offered to Krishna. It is very nice that all the men should participate in deity-worship, this will sanctify their all other work. I think you are the most clever manager, better than your husband, because you are organizing temple routine very nicely for serving the deities and this will have very good results for everyone.
I hope this meets you in good health and cheerful mood.
Your ever well-wisher,
A.C. Bhaktivedanta Swami

Letter to Himavati Delhi Nov 17 1971

Prabhupäda said he was glad to meet him and asked him, “What are you growing?” The gardener eagerly showed Prabhupäda some of the plants and fruits he was raising in the greenhouse, including watermelons and varieties of flowers. He also pulled out trays from underneath a greenhouse table and showed Prabhupäda his mushrooms.
“Oh, we do not eat this,” said Prabhupäda. “This is fungus.” The man admitted that it was a fungus. Prabhupäda explained that mushrooms do not have a good taste, and because they grow in a dark damp place, they are considered food in the mode of ignorance. Srila Prabhupäda then suggested that the gardener should try to grow ladyfingers, but the man didn’t know what Prabhupäda meant. Prabhupäda pointed to his own fingers. “You should grow these ladyfingers.” He gave the Hindi word, bhindi, which the man also couldn’t understand. Finally the gardener understood that Prabhupäda was talking about okra. Prabhupäda asked if the man could grow mangos, but he said he couldn’t, not even in the greenhouse.

This is from a book called Srila Prabhupada Nectar

I could not find the reference but apparently Srila Prabhupada sanctioned one temple and this one temple only, the temple in Paris, that they could offer mushrooms. This was told to be by a Prabhupada disciple Caitanya Candra Prabhu but I could not find any reference regarding this matter. It was also mentioned that the mushrooms there were not grown in dark places and there were a very large local customary dish.

I would like to also mention that from my understanding there is difference in what can be offered at the temple and what can be offered and home, especially in the case of new young devotees.

For example pasta is not offered in the temple unless the noodles are home made but it is regularly offered at home by devotees. Devotees often at home cook things like vegetarian sushi, such meals often have soy sauce, which is not offerable in the temple, but however it is accepted to be offered at home.

The items that are never to be offered are meat, fish, eggs and other non veg foods and Garlic and onions. These items are never accepted by Krishna or Guru by whatever methods.