I'm assuming you created a child theme based on boost, if not this is a great place to start:
https://docs.moodle.org/dev/Creating_a_theme_based_on_boost
That being said the tutorial only goes so far. Not everything can be done with mustache files yet although I believe they continue to develop boost dropping more and more into templates.
https://docs.moodle.org/dev/Templates
If you are used to traditional theme dev then you should be no stranger to overriding a renderer:
https://docs.moodle.org/dev/Overriding_a_renderer
Drop this into /moodle/theme/your_theme/classes/core_renderer.php
class theme_your_theme_core_renderer extends \theme_boost\output\core_renderer {
public function heading($text, $level = 2, $classes = null, $id = null) {
global $PAGE;
if (strpos($PAGE->pagetype, 'mod-') === 0) {
if ($level == 1) {
if (($tmp = strstr($PAGE->title, ': ')) !== false) {
$text = substr($tmp, 1);
}
} else if ($level == 2) {
return;
}
}
return parent::heading($text, $level, $classes, $id);
}
}
If you want to target different modules you can change "mod-" to something like "mod-book".