In a food processor, process the onion until smooth. Add the tarama and process until well combined. Fill a bowl with water and dip the bread slices, 1 at a time, into water and squeeze any excess water. Add the bread to the tarama mixture, 1 slice at a time, and blend, alternating with additions of the oil, until almost smooth. Add the lemon juice and process 30 seconds. Garnish with scallions and serve.
Makes about 4 cups