Managed to get it working with the following:
$recipes_from_title = Recipe::get()->filter('Title:PartialMatch', $searchString);
$recipes_from_diet = RecipeType::get()->filter('Title:PartialMatch', $searchString)->relation('Recipes');
$matching_recipes = new ArrayList();
foreach($recipes_from_title as $recipe_from_title) {
$matching_recipes->push($recipe_from_title);
}
foreach($recipes_from_diet as $recipe_from_diet) {
$matching_recipes->push($recipe_from_diet);
}
return $matching_recipes;