An application after verdict by a trial judge sitting alone to reopen the defence case or to declare a mistrial based on new evidence should not be approached as an appeal of the trial judge’s own decision.
Keep in mind also that (in a judge alone trial) once the sentence is imposed the judge’s authority is spent (functus officio).
When faced with an application to reopen the evidence, the trial judge should first be satisfied that the proposed evidence is relevant to a material issue in the case [see item #2, The Test to Reopen After Conviction, below]. That determination can usually be made on the basis of counsel’s summary of the anticipated evidence.
R. v. Hayward (1993), 86 C.C.C. (3d) 193 (Ont. C.A.) at para. 17
The test for reopening the defence when the application is made prior to conviction is different from the more rigorous post-conviction test.
The Test to Reopen After Conviction
The legal test to reopen the defence after conviction tracks the test for admissibility of fresh evidence on appeal set out in Palmer v. The Queen,  1 SCR 759, 1979 CanLII 8 (SCC):
1. the evidence should generally not be admitted if, by due diligence, it could have been adduced at trial although this general principle will not be applied as strictly in criminal cases as in civil cases;
2. the evidence must be relevant in the sense that it bears upon a decisive or potentially decisive issue in the trial;
3. the evidence must be credible in the sense that it is reasonably capable of belief;
4. it must be such that if believed it could reasonably, when taken with the other evidence adduced at trial, be expected to have affected the result;
R v Kowall, 1996 CanLII 411 (ON CA), leave denied  SCCA No 487;
R v Palmer, 1979 CanLII 8 (SCC).
5. In addition to the Palmer criteria, a trial judge must consider whether the application to reopen is in reality an attempt to reverse a tactical decision made at trial. Counsel must make tactical decisions in every case. Assuming those decisions are within the boundaries of competence, an accused must ordinarily live with the consequences of those decisions.
Kowall at para. 32; R v Arabia, 2008 ONCA 565 (CanLII) at para. 46.
As a general rule, permission to reopen would be followed by setting aside the prior finding(s) of guilt, reception of the further evidence, together with any evidence offered by the prosecutor in reply, the submissions of counsel, and a decision on the adequacy of the prosecution’s proof in light of the new evidence. In some instances, it may be necessary for the court to declare a mistrial. However, the authority to declare a mistrial should only be exercised in the clearest of cases.
Arabia at paras. 49, 52.
Reopening Sentencing Proceedings
The above principles require some adaptation for applications to reopen sentencing proceedings to introduce fresh evidence relating not to the offender’s guilt or innocence, but to matters relevant to the sentencing principles yet to be applied.
See R. v. E.S., 2017 BCCA 354 (CanLII), at para. 25.