fuse-angular/src/assets/angular-material-examples/select-optgroup/select-optgroup-example.html

<mat-form-field>
    <mat-select placeholder="Pokemon" [formControl]="pokemonControl">
        <mat-option>-- None --</mat-option>
        <mat-optgroup *ngFor="let group of pokemonGroups" [label]="group.name"
                      [disabled]="group.disabled">
            <mat-option *ngFor="let pokemon of group.pokemon" [value]="pokemon.value">
                {{ pokemon.viewValue }}
            </mat-option>
        </mat-optgroup>
    </mat-select>
</mat-form-field>