The phrase "at once substantial" is correct and usable in written English.
It can be used to describe something that is significant or considerable in nature, often implying that it is both immediate and important.
Example: "The report provided at once substantial evidence of the project's success and a clear path for future improvements."
Alternatives: "immediately significant" or "simultaneously important".
At the same time, the fact that Mr. McHugh's departure would at least temporarily reduce New York's once substantial Republican delegation in the House to a total of two members was a vivid reminder of the party's steep decline in the Northeast.
A major 1998 law governing village elections ducked the central unresolved issue, at once granting substantial authority to elected committees and noting the leading role of the party branch.
Once substantial decline has occurred, exercise-training is not effective at reversing the trend.
The church itself is surprisingly small, and gives the odd impression of being at once rickety and substantial.
The crepe itself, made with buckwheat flour, was at once light and substantial, its faintly toasted flavor balancing the mussels' marine tang.
Ms. Hyland, with scant space to work with, has admirably made the museum's homage feel at once tender and substantial.
Examples of the kind are given by the learned commentator; and he states that verbal reflections upon the chastity of an unmarried female are not actionable, unless they have prevented her from marrying, or have been accompanied by special damage; but, if they are published in a newspaper, they are at once actionable, and substantial damages are recoverable.
At once giddy and substantial, "Funny Bones," which makes an inspired use of expertly staged flashbacks, is distinctive in the manner of Richard Rush's "The Stuntman," a film that it oddly recalls.
Thus the simulation study showed that the gain of using information from all arrays all at once by TRANSMODIS was substantial.
