Learn Croatian: Bosiljak je bolji uz svježe rajčice. - Basil is better with fresh tomatoes.

While s (with the instrumental case) means 'with' in the sense of physical accompaniment, mixing, or using a tool, uz (with the accusative case) means 'alongside' or 'in combination with'. When talking about food pairings that complement each other rather than being physically mashed together, uz is the most natural choice in Croatian.

What case follows the preposition uz in this sentence?

The preposition uz takes the accusative case here. It forces the phrase svježa rajčica (fresh tomato) into the accusative plural, which becomes svježe rajčice.

Since svježe rajčice is also the nominative plural form, how can we be sure this is the accusative case?

You are right that for feminine nouns ending in -a, the nominative plural and accusative plural look exactly the same (both end in -e). We know it is accusative purely from the grammatical rule that the preposition uz requires the accusative case when used to mean 'alongside' or 'with'.

Why does the word for 'better' take the form bolji here instead of the more common bolje?