-1
Just use the linear-gradient on the diagonal does not have much secret
body {
  background-color: azure; 
}
.box {
  width: 100px;
  height: 100px;
  border: 7px solid #666;
  border-radius: 50%;
  background-image: linear-gradient(to top left, transparent 47%, red 48%, red 53%, transparent 54%)
}
<div class="box"></div>

Have you tried anything? What was the result?
– Woss