Postgis does not support selecting feature collections, thus I had to build one on my own. This is the resulting query I used:
SELECT json_build_object( 'type', 'FeatureCollection', 'properties', json_build_object(), 'features', json_agg(st_asgeojson(field.geometry)::json) ) FROM field WHERE farm_id = 'XYZ';
Useful postgis functions here are
json_build_object() which creates a new object and
json_agg() which creates a new array.
Other json operations can be found in the postgres documentation.