Learn Dutch: Deze verse brie is tamelijk populair. - This fresh brie is rather popular.

You can use the diminutive brietje. As you are learning in this lesson, adding a diminutive ending to a mass noun turns it into a countable portion. So, while brie refers to the cheese in general, een brietje means a specific piece, wedge, or small wheel of brie.

Why does verse get an -e at the end, but populair does not?

Verse is placed directly before the noun it describes, so it gets the standard -e ending. Populair is separated from the noun by the verb is (predicative use), and adjectives in this position never take an -e ending. Also, notice that the root word for fresh is vers; unlike words like huis that change s to z, the s stays an s in verse.

What exactly is the nuance of tamelijk compared to heel or erg?

Tamelijk translates to "rather," "quite," or "fairly." It expresses a moderate degree of something. It is stronger than "a little bit" (een beetje) but noticeably weaker than "very" (heel or erg).

Could I use dit instead of if I am pointing at a specific physical piece of cheese?