Flash a 'virgin' ATMEGA328PU with the Arduino Bootloader. On MacOS simply brew install avrdude --with-usb

arduino.sh

#!/bin/sh
OPTS="-p m328p -c avrisp2 -P usb"
PWD="$(pwd)"

# Unlock the chip
avrdude $OPTS -U lock:w:0x3f:m

# Set the Fuses
avrdude $OPTS -e -U lfuse:w:0xff:m -U hfuse:w:0xda:m -U efuse:w:0x05:m

cd ~/Applications/Arduino.app/Contents/Resources/Java/hardware/arduino/bootloaders/atmega
pwd

# Write the Bootloader
avrdude $OPTS -e -U flash:w:ATmegaBOOT_168_atmega328.hex:i

# Lock the chip
avrdude $OPTS -e -U lock:w:0x0f:m

cd $PWD
pwd