Open the first attached expansion panel in an Angular Material accordion

You can use first variable, I made an stackblitz, you can see here.

You can do as this way:

<mat-accordion class="example-headers-align">
  <mat-expansion-panel *ngFor="let item of [1,2,3]; first as isFirst" [expanded]="isFirst">
    <mat-expansion-panel-header>
      <mat-panel-title> {{item}} </mat-panel-title>
    </mat-expansion-panel-header>
    {{item}}
  </mat-expansion-panel>
</mat-accordion>

You can see here the use of the *ngFor variables for more information.