Learn Russian: Эта котлета очень вкусная.

Not exactly! While it sounds like "cutlet," a Russian котлета is usually a patty made of minced meat (like a hamburger patty or rissole), whereas an English cutlet is typically a whole piece of meat, often on the bone.

Why is there no word for "is" in this sentence?

In Russian, the present tense of the verb "to be" is almost always omitted. You simply put the subject (эта котлета) directly next to the adjective that describes it (очень вкусная).

Could we use the short adjective вкусна instead of the full adjective вкусная here?

You could, but it sounds much more formal or poetic. For everyday speech, especially when talking about something ordinary like a food item, Russians naturally use the full adjective вкусная.

Why do we use Эта instead of the default Это?

Here, "this" is attached directly to the noun, acting as an adjective. Because котлета is a feminine noun, it requires the feminine form Эта. You would use the default Это if you were making a general statement like "This is a tasty rissole" (Это очень вкусная котлета).