You are correcte in noticing that "ton" is the masculine form while "assiette" is feminine. This apparent mismatch is due to the word assiette starting with a vowel. "Ta assiette" would have two vowels clashi g in a difficult to pronounce combination.
The French take the simple way out and use the masculine form whenever this happens.
"Mon amie" is another example. "Amie" is female but you use "mon" nonetheless I believe there is a dedicated lesson on this.
-- Chris.
