an onion seed germinates and a single leaf emerges from the soil. is the onion a monocot or a dicot? What ?

Monocotyledon. The fact that it only sprouts one leaf makes it a monocot.

The seeds most often used to show the difference between Monocotyledons and Dicotolydons is corn and beans.  Dicots are not easily separated.  The Dicots siparate into two cotyledons.  Peanuts are another good example. The second thing after seed inspection in determining a monocot from a dicot is its emerging seedling.  The monocot sends up a single leaf.  The dicot sends up a single shoot with two dicot leaves.  The second set of leaves that appear on a dicot are called the "True Leaves". The next outward appearance of the leaf that further differentiates the two is their vein webbing.  The monocot, typical of the grasses, has veins that fun from the bottom to the top with out branching. Onion. The veins on a dicot are branched and form a web or netting. Oak.or beans. The flowers typically are segregated by their petal and internal designs. The monocot are three  or multiplys of three. The dicot is divided either in fours or fives.  This is enough differentiating to give you an excellent start at identifying Monocotyledons from Diccotyledons.  Good Luck.