First of, I’m a learner of the language like you, so I apologize for any mistakes, but I’ll try to answer your question. In this sentence, の is being used as a possessive particle. The translation would be:
きょう の おすすめ は なんですか。= What is today’s recommendation?
きょう の おすすめ = Today’s recommendation.
If you wrote it as you said, with は and が, it would translate slightly differently:
きょう は おすすめ が なんですか。= Today (as opposed to other days) what is recommended (or what is your recommendation)?
The first part inside the parenthesis highlights what the は particle is doing in this case, and the second part in parenthesis is just an optional translation since the meaning can change based on the context. As you may, or may not know, in Japanese it is normal to drop pronouns until a distinction is needed. Since I don’t have any context to go from, I put two possible translations.
In any case, の is just a possessive here. That’s the short answer :-P.
Thanks for clearing that up for me. Overthinking the smaller details always seems to give me the most trouble. I'd like to double-check something if you don't mind answering another question related to the の particle.
Since の modifies a noun into an adjective, is the の particle still required if an actual adjective like 'big' (おおきい) or 'beautiful' (うつくしい) is used?
I'm assuming that if the word ends in い then those words are I-adjectives and の is dropped, and if they're nouns that need to modified then の is used instead, correct?
As for your other question, your thinking is correct. The particle の is not needed for most い adjectives, except for 少ない (sukunai) and 多い (ooi). However, you need to first convert them to adverbs before tagging the の particle to them. This is better explained in the following article: