Learn Portuguese (Portugal): Eu coso sempre a bainha lenta e cuidadosamente. - I always sew the hem slowly and carefully.

When two or more adverbs ending in -mente are joined together by a conjunction like e, we only keep the -mente suffix on the very last adverb. The first one drops it to avoid clunky repetition, so lentamente e cuidadosamente becomes lenta e cuidadosamente.

If lenta is acting as an adverb here, why is it in the feminine form when there is no feminine noun to agree with?

Adverbs ending in -mente are always built by adding the suffix to the feminine singular form of the adjective. When you drop the suffix for the first adverb, you are left with that feminine base form (lenta), which stays feminine regardless of the gender of any other words in the sentence.

I thought coso was the verb for cooking or baking. Am I mixing it up?

You are thinking of a very tricky pair of homophones! This is coser (with an s), which means to sew. The verb you are thinking of is cozer (with a z), which means to cook, boil, or bake. In European Portuguese, they are pronounced exactly the same way.

Could I put the adverb before the verb and say Eu sempre coso?

In European Portuguese, adverbs of frequency like sempre normally go immediately after the verb in a standard affirmative sentence. Putting sempre before the verb is grammatically possible but strongly sounds like Brazilian Portuguese to native ears.