The phrase "asserting in a" is correct and usable in written English.
It can be used when introducing a statement or claim that is being made in a specific context or manner.
Example: "The researcher is asserting in a clear and concise manner that the results support the hypothesis."
Alternatives: "claiming in a" or "stating in a".
BANGKOK — Prime Minister Samak Sundaravej went on the defensive Thursday, asserting in a nationwide radio address that he would not resign despite mounting pressures against him.
"He thought that might be a good place to seek out business opportunities," said his father, Mikell Hagood, asserting in a telephone interview that his son had not been a willing guest of the Taliban.
The central bank disputes this, asserting in a July statement that the Bank of Cyprus had been "fully recapitalized" thanks to the forced deposits-for-shares conversion and will now be able to "support the Cyprus economy".
A three-judge panel of the United States Court of Appeals for the Second Circuit overturned the death sentence, asserting in a 2-to-1 ruling that federal prosecutors had violated Mr. Wilson's constitutional rights.
But he denied that he was a "popularizer," asserting in a chiding letter to Time magazine in 1939: "I really have no thought of 'popularizing' history in the sense of oversimplifying it for simple minds.
"On the contrary, by asserting in a prominent place (the journal Nature) that this non-existent risk needs solution, these papers subtract from the ability of the body politic to do sensible risk assessment to create sensible public policy.
Hamling consciously adopted an editorial policy oriented toward entertainment, asserting in an early issue that "science fiction was never meant to be an educational tour de force".
"It won't happen," Mr. Hennessey asserted in a blog post last week.
But in one such case, rights to a body were asserted in a particularly startling way.
Thank you!" President Trump asserts in a tweet after new Cohen and Manafort case filings.
Apparently: Peter Bradshaw of this newspaper asserted in a five-star review that it is "magnificent".
