So, the concept goes like this:  
Gnomes

1. Good Int stat
2. Bad movement speed
3. Rock Gnomes have "artificer history expertise"

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Artificers

1. Can use Booming Blade with a quarterstaff or a Warhammer since they have Versatile
   1. You just need a 1-handed weapon or get the War Caster feat to use the  weapon's higher damage dice
      1. This would actually be an advanced version of the build since you can cast Booming Blade as a reaction to deter enemies from running away from yourself
2. The Steel Defender is immune to Exhaustion, so it can literally Dash all day and not need to slow down
   1. 80ft/second=54.5 mph (87.8 kph)  If you use Haste, that's 1 minute of double speed, which comes around to just under 2 miles (2.92 km)

Mounts

1. Can disengage, keeping the enemy from getting an Attack of Opportunity

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So, just use your Steel Defender as a mount.  Use your Bonus action each turn to command your Steel Defender to Disengage with 40 feet of movement, forcing the enemy to either Dash to catch up to you (if their movement speed is <40 feet per round) or not move to prevent the damage from Booming Blade.  

BONUS ROUND!!

Make a special "Upside down compatible" saddle for your mount and cast Spider Climb for even more movement options

Yes, a gnomish battlesmith rinding on his Iron Defender is absolutely a possible character design. I even made one for a character challenge once, if I may toot my own horn here ;)

Also, You can use Booming Blade with a 2 handed weapon or while using a weapon 2 handed, since it doesn't have any somatic components and the only material component you need to be holding is the weapon itself.

That said, since the Battlesmith gets Extra Attack at lvl 5 (which doesn't combine with Booming Blade) it may not always be the best option for an battlesmith artificer.

Yep now i will be making a gnomish army of artificers to guard the gnomish city. this is a great idea and i love the idea of dipping into fighter for cavalier.