How can a SIGNIFICANT AMOUNT of onion juice be obtained?
Initially, this seemed a logical question... yet, notice this healthy, freshly-sliced onion. What you are seeing is pure onion juice. Of course, not every onion will be as fresh and as tender and as healthy, but it hints at juice beyond.
So, for small amounts of juice, simply grate and sieve a portion of the onion. (Actually, some people prefer using their garlic presses.)