Right side 斗 is the indexing radical, picturing a ladle or dipper. The connection to 'slanting' comes from the way a dipper tilts when pouring - held at an angle so its contents flow out. That image of held-on-the-tilt is the semantic core of 斜.
Left side 余 supplies the sound - yú shifted to xié in 斜's reading. The 'surplus' meaning is dormant; treat 余 here as a phonetic. Same series: 涂 to smear, 途 path, 除 to remove - all share the basic yú root with various readings.
