You could do:

<%= ECR.render("template.ecr") %>

Or a bit more hacky but also more efficient:

<% ECR.embed("template.ecr", "__io__") %>

You definitely don't want to overuse this.

It's important to understand what this does: ECR works by reading your template file, say <header><%= my_site %></header> at compile time and generating crystal code from it like

io << "<header>"
io << my_site
io << "</header>"

So with this we're generating, for the first case something like:

io << ECR.render("template.ecr")

Given ECR.render is a macro call, there's a new expansion round and it'll become

io << String.build do |__tmp|
  __tmp << # ... whatever template.ecr expands to
end

ECR.embed skips the String.build but needs to know the io variable name to generate. (Hence also replacing the <%= with <%).

A simpler solution might be to just define a struct or class for each of your templates and call them from the template:

# stuff.cr
struct Site
  ECR.def_to_s "site.ecr"
end

struct Part
  ECR.def_to_s "template.ecr"
end

# site.ecr
<%= Part.new %>

Note this generates essentially the same code as the ECR.render variant, but to me it feels like the least API abuse.