The price of onions, a major issue in India's national election, fell today in the wake of Indira Gandhi's landslide victory.

That is a matter of no small importance in India, where nothing can be cooked without onions and where a recent sharp jump in their price was front-page news in all papers here.

Gandhi, whose Congress-I (for Indira) Party is on its way to controlling close to 75 percent of the votes in the lower house of Parliament, had run big newspaper ads blaming her election rivals for causing the increase in the price of onions.

Today the price dropped in markets here from about 25 cents a pound to 20. The price of cooking oil, another domestic staple, also fell.

Observers speculated that traders trimmed their prices to forestall strict controls that the new Gandhi government might impose. It is likely to take office early next week.

Gandhi appeared to be rolling up the most impressive parliamentary majorities since her father, India's first prime minister Jawaharlal Nehru, ran for office in 1952 as a hero of the fight for independence.

Late reports showed her Congress-I Party winning 341 of the 498 seats declared. There are 542 seats in all. Her nearest opponent, Caretaker Prime Minister Charan Singh's Lok Dal Party, has won only 39 seats.

Jagjivan Ram's Janata coalition, which was expected to give Congress I a real fight, is finishing a distant third with just 29 seats so far.

The election destroyed the myth that the 71-year-old Ram, a venerable politician who has been in almost every Cabinet since independence, controlled the votes of India's 100 million untouchables. Although a member of that caste himself, Ram showed that most untouchables' votes stayed with Gandhi.

The strength of the Hindu nationalist Jan Sangh Party, a major part of the Janata coalition, also proved to be illusory. The Jan Sangh organization was believed to be strong in the cities, especially New Delhi, but it only captured one seat here and 16 of the 110 seats from urban areas.

This collapse of the Janata coalition, which defeated Gandhi in 1977 over the issue of harsh emergency rule, was one of the most surprising aspects of the election. Even the magazine India Today, which predicted on the basis of polls that Gandhi's party would win a clear majority in the Parliament, said the Janata would finish a close enough second to be a viable oppostion.

Instead, it appears that it will fall behind the Lok Dal, a Janata spinoff that has only the parochial support of the small landowning farmers of two north Indian states.

While party leaders such as Ram and Charan Singh won reelection, it appears that a number of familiar faces were let loose by their constituencies. a

At the same time, Congrees-I, hit by defections during the past 33 months when it appeared that Gandhi was finished as a political force, ran a large number of first-time candidates.

Many of them were reported to have been handpicked by Gandhi's 33-year-old son Sanjay on the basis of their loyalty to him and his mother. According to widespread reports here, he was responsible for selecting between 80 to 180 candidates, including most of those who upset the Janata Party in Delhi.

One of the winners known to be close to Sanjay, for instance, is Chranjit Singh, a newcomer to politics who controls the company that makes Campa Cola, the Indian imitation of Coca-Cola.

"This is not the same Congress-I Party that was thrown out of office three years ago," said a Western diplomat who keeps a close eye on Indian domestic politics. "Many of Mrs. Gandhi's most prominent association men, like Jagjivan Ram, Y. B. Chavan and Charan Singh, have broken with her. Just look at the Cabinet roster of the opposition parties."

This means, most observers here believe, there will be a number of new faces in the Gandhi Cabinet.