Learn French: Mon oncle est un très bon jardinier. - My uncle is a very good gardener.

You are right that we normally drop the article for professions, such as saying Il est jardinier. However, when the profession is modified by an adjective like très bon, the rule changes and we must bring the article back. So it becomes un très bon jardinier.

Why does bon come before jardinier instead of after it?

Most French adjectives come after the noun, but bon belongs to a small group of common adjectives that go before the noun. A helpful memory trick for these exceptions is the acronym BANGS (Beauty, Age, Number, Goodness, Size). Since bon describes goodness, it is placed right before jardinier.

Are there any required liaisons when pronouncing this sentence?

Yes, there are two! You must make a liaison between mon and oncle (sounding like mo-noncle) and between est and un (sounding like è-tun). This keeps the sentence flowing smoothly without pausing between vowel sounds.