Great, glad it helped! Yeah it's hard to pick a word that works so go for anything that has a letter different. so menu mend works and the -as is just the grammar suffix. I find my English brain gets stuck on -ing words when that's really just a suffix and if taken away what am I really left with? The rest is usage. Good luck!
This Esperanto word for "more" is not an adjective (Those are the ones that end in -a.) I found "plia" and it means "another" or "one more" and here he is ordering more than just one more. Also, some adverbs end in -e, but not all of them. http://en.lernu.net/lernado/gramatiko/konciza/adverboj.php http://esperanto-panorama.net/vortaro/eoen.htm
I have seen another version that ends in -u, so I am more confused as to when we use "pli" and when we use "plu". "plu" means "else", "further", "more", "on" There is also "plua" which means "additional" or "another". So I am with you a bit, I wonder if we could say "plua tomatoj" or do we still need to add "da"'
I think the problem is that the English word "more" does not change form, but is an adjective, adverb, pronoun and even is used as a noun. Scroll down at this site: http://www.merriam-webster.com/dictionary/more
When you say "He ordered more of the tomatoes", "more" is the direct object and "tomatoes" is the object of the preposition "of". The prepositional phrase is adjectival as it describes "more" which is being used as a noun or pronoun.