To represent the number of cups of diced onions based on the number of cups of chopped tomatoes (denoted by \( t \)), we start with the information that the number of cups of chopped tomatoes is twice that of the diced onions.
Let's denote the number of cups of diced onions as \( d \). The relationship provided in the recipe can be expressed as:
\[ t = 2d \]
To express \( d \) in terms of \( t \), we can rearrange this equation:
\[ d = \frac{t}{2} \]
Thus, the expression that can be used to represent the number of cups of diced onions based on the number of cups of chopped tomatoes \( t \) is:
\[ \frac{t}{2} \]