They're the same in their basic conceptual content (or ideas) but different in their construal (or how those ideas are interpreted or presented). Generally speaking, beginning the expression with "לאבוקדו" creates a 'topical frame of reference' for the comment that follows (about the avocado)—namely, that it "has [or there is to it] a taste of onion": "יש טעם של בצל". So "לאבוקדו" is the topic of the clause, in this instance—what it is about. Moving it to the front simply draws more attention to it and places it at the center stage (as the topic) for the main communicative point (or comment) that follows.
Some linguists treat these issues of "information structure" in depth, for example, with respect to biblical languages and translation (e.g., Randall Buth, Steven Runge, and Stephen Levinsohn). It's a big complex area of study more generally in the field of linguistics.
No. yesh l- is how you state something "has" something in hebrew so yesh-la-avokado (יש לאבוקדו) means "the avocado has" - "a taste of onion" (note: yesh-le-avokado with an E means "an avocado has..." HA-avokado... would mean ("The avocado, there is a taste of onion") since "yesh" is effictively unmodified
You actually paid for the course? It's free isn't it? I've been to Israel 8 times and have been studying for as many years. My Hebrew isn't perfect... :) but then again, neither is my English. I also teach Hebrew for a living, not at like a technical linguistic level. But enough of the daily lingo to bring someone to fluency. Feel free to friend me on here and message me with questions, I love helping people learn Hebrew.