Blending onion and garlic together will alter their individual flavors, creating a more pungent and combined taste. It's generally better to add them separately to control the flavor profile.
Onion and garlic, while both alliums, have distinct flavor compounds. When you blend them together, you're essentially forcing these compounds to interact more intensely than they would if cooked separately. This interaction results in a more concentrated and sometimes harsher flavor. The enzymes released during blending also accelerate the breakdown of these compounds, leading to a quicker release of their aromas and flavors. Furthermore, the texture changes significantly. Blended onion and garlic become a paste, which cooks differently than diced or minced pieces. This altered cooking process also contributes to the change in flavor. For most recipes, adding onion and garlic at different stages allows you to build layers of flavor and achieve a more nuanced result.
If you're short on time and need to use a blended onion-garlic mixture, consider sautéing it gently over low heat for a longer period. This will help mellow out the harshness and develop a sweeter, more balanced flavor.