When was the last time a major battle was fought by two opposing armies on an open field?

Your question is pretty vague. I think perhaps you are asking about the last battle fought with Napoleonic tactics. Where large groups of men would be used as a unit rather than as individual soldiers. If that is your question I suggest you ask it as something like. "When was the last time Napoleonic tactics were used in warfare?"

If you really just want to know when the last time two armies met on an open field I think the answer would be the Battle of 73 Easting. Perhaps not a field but pretty wide open and flat. How wide open and flat? You might ask. It was so flat they didn't have a major landmark to name the battle after so they named it after GPS coordinates. The battle has had a major impact on war planning over the last 20 years as it demonstrated the superiority of technology on the battlefield. US forces used equipment about 20 years more advanced than the Iraqi forces they faced. The result of the battle was an overwhelming victory for the US forces.